Unify package doc and github README.md (#72)

MOTIVATION

Prior to this set of changes, two sets of documentation had to be maintained:
the GitHub README.md and the Go package documentation in doc.go.  The README.md
file is a superset of documentation that includes the following items:

- Various badge icons (build, godoc, coverage)
- Getting started guide with sample applications
- Comparison to other CLI tools
- Installation instructions
- A copy of the package doc.go
- License information

Because a copy of doc.go was included in README.md, over time these two versions
have diverged.  Committers updating README.md would forget to submit the same
patches to doc.go and vice versa.  Further complicating the situation is that
GitHub markdown is more featureful (multi-level headings, lists, etc ...) than
the simple and straightforward Go doc standards.  Consequently, the package
documentation (doc.go) contained within README.md acquired additional markup,
further cementing the need to maintain two slightly different versions of the
package documentation.

Due to the above issues, contributing to the project's documentation was not an
easy task. If done properly, a contributor would have to update the README.md
document and then update the doc.go package documentation stripping out the
markdown.  This assumes the contributor was editing a portion of the docs that
had not diverged over time.  In that case, additional work was required to
synchronize the documents.

CHANGES

This commit includes several changes to improve the overall documentation of
the mow.cli project. Below is a short summary of each:

- To eliminate the need to maintain two sets of nearly identical docs, the
  README.md file is now autogenerated using the Go autoreadme package. This
  tool generates a README.md from a README.md.template file. The template
  file includes all of the original README.md content (described above), but
  also includes placeholders where the contents of doc.go are included when
  the file is generated.

- A new 'doc' target was added to the Makefile, which generates README.md for
  inclusion into the repository for GitHub.  No changes should ever be made
  directly to the README.md file anymore as they will be overwritten when
  'make doc' is executed.  All changes should either be performed in
  README.md.template or doc.go.

- Extensive edits to the Go package documentation, doc.go, were made to improve
  readability. This included numerous grammatical changes, rephrasing of
  sentences, ensuring consistency of examples, and rearrangement of the sections,
  so the docs were more accessible to new users.

               Old Ordering              New Ordering
               ============              ============
        	- Basics                  - Basics
        	- Options                 - Options
        	- Arguments               - Arguments
        	- Operators               - Operators
        	- Commands                - Commands
        	- Custom Types            - Interceptors
        	- Interceptors            - Exiting
        	- Spec                    - Spec Strings
        	- Spec Grammar            - Spec Grammar
        	- Default Spec            - Default Spec
        	- Exiting                 - Custom Types

- Extensive reformatting of doc.go to ensure compliance with the basic Go doc
  style recommendations.  This required eliminating extraneous headings and
  lists (bulleted and enumerated).  Documentation can still be well-written
  without these constructs.  By adhering to the Go doc guidelines, when the
  package documentation is included by autoreadme in the README.md, it looks
  natural and fits in with the overall look and feel of the original README.md
  document.  One of the biggest challenges was dealing with the lack of multi-
  level headings in Go do.  With only a single level heading, its use must be
  frugal.  The prior README.md, however, made liberal use of sub-headings
  especially in the original Spec section.  I simplified that entire section
  by writing each of the subsections as a separate paragraph with a single
  example, which maintains a rhythm for the reader making it flow as well,
  or perhaps even better, than that original README.md.

- An additional example application called "A Larger Multi-Command Example" was
  written to show an alternate way of specifying the command structure where
  all commands are defined together in a section, but their implementation is
  deferred to functions. As a newcomer evaluating various CLI tools, I initially
  dismissed mow.cli because I did not like the style of defining every command
  in a single function as portrayed in the examples contained within README.md.

Makefile

@@ -14,6 +14,9 @@ lint:
 vet:
 	go vet
 
+doc:
+	autoreadme -f
+
 fmtcheck:
 	@ export output="$$(gofmt -s -d .)"; \
 		[ -n "$${output}" ] && echo "$${output}" && export status=1; \
@@ -28,6 +31,7 @@ setup:
 	go get golang.org/x/tools/cmd/cover
 	go get github.com/mattn/goveralls
 	go get github.com/modocache/gover
+	go get github.com/divan/autoreadme
 	go get -t -u ./...
 
 .PHONY: test check lint vet fmtcheck ineffassign