✨ Add low-setup usage

[CLiu13]

This adds the ability to use moban in an ad-hoc manner
without a config file. This also adds environment variables
as a fallback data source if the default/specified data files
do not exist.

Closes #133

Before raising the PR, here is a check list:

1. have you written unit tests for your code changes?
2. have you run "make format"?
3. are you requesting to "dev"?
4. have you updated the change log?
5. do you think that you can understand your changes after 6 month?
   5.1) can someone else understand your changes without your explanation?
6. are you pround of your code changes?
   6.1) do you have the feeling of achievement?
7. please add your name and github link to contributors.rst in alphabetical order.

[CLiu13]

Working on writing tests and updating the proper documentation.

[codecov-io]

Codecov Report

Merging #146 into dev will decrease coverage by 0.62%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##              dev    #146      +/-   ##
=========================================
- Coverage   98.33%   97.7%   -0.63%     
=========================================
  Files          39      40       +1     
  Lines        1857    1918      +61     
=========================================
+ Hits         1826    1874      +48     
- Misses         31      44      +13

Impacted Files	Coverage Δ
moban/reporter.py	98.38% <100%> (+0.2%)	⬆️
moban/_version.py	100% <100%> (ø)	⬆️
...sts/integration_tests/test_command_line_options.py	98.25% <100%> (-1.75%)	⬇️
tests/test_engine.py	100% <100%> (ø)	⬆️
moban/mobanfile.py	99.19% <100%> (-0.81%)	⬇️
tests/fixtures/environ_vars_as_data/test.template	100% <100%> (ø)
moban/plugins.py	91.44% <55.55%> (-4.91%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e0880fb...d9e98b8. Read the comment docs.

[chfw]

An update on the documentation regarding this new usage would be helpful.

[jayvdb]

With a rule foo/bar.py: foo/bar.py.jj2

moban -o foo/bar.py -t foo/bar.py.jj2 does find the relevant entry in the config and only process that one entry.

However ... moban -t foo/bar.py.jj2 ...

Traceback (most recent call last):
  File "/home/jayvdb/.local/bin/moban", line 11, in <module>
    load_entry_point('moban', 'console_scripts', 'moban')()
  File "/home/jayvdb/projects/moban/moban/moban/main.py", line 36, in main
    count = handle_moban_file(moban_file, options)
  File "/home/jayvdb/projects/moban/moban/moban/main.py", line 121, in handle_moban_file
    mobanfile.handle_moban_file_v1(moban_file_configurations, options)
  File "/home/jayvdb/projects/moban/moban/moban/mobanfile.py", line 40, in handle_moban_file_v1
    target = extract_target(command_line_options)
  File "/home/jayvdb/projects/moban/moban/moban/mobanfile.py", line 149, in extract_target
    "Please specify a output file name for %s." % template
Exception: Please specify a output file name for foo/bar.py.jj2.

And moban -o foo/bar.py processes all rules. I think this is an important mode of operation, as people will often know the filename of the file they want updated, and may not remember which template it is.

And all of the above then process the copy:, which should be omitted if a specific rule was requested on the CLI. Note I've been think the copy: rules should be implemented as a builtin plugin template type 'copy', so that they are no longer a special set of targets.

[chfw]

However, I think the code refactoring could be done in a separate PR. Letting this PR pending so long is not good.

[chfw]

Code refactoring is required before releasing. Otherwise, functionalities are agreed.