Add Travis CI configuration

- Install basic dependencies for reading, parsing, and writing markdown
  files
- Setup index module with `bumpVersion` function that configures and
  uses a pipeline witha custom plugin

Duplicate the "Unreleased" header and link definition and apply the new
version to the link label.

- Add `jasmine` as a development dependency
- Run `jasmine init`
- Set `jasmine` as the `npm test` command

Add test suites for the plugin's attacher and transformer function.

Using failing tests as a guide, fix issues with the `unified` plugin.

- Validate the `version` property passed into the attacher
- Validate the tree passed into the transformer
- Handle nodes without children
- Update the URLs for the unreleased and bumped link definitions

- Add argument matchers for `unist` nodes
- Add argument matchers for `vfile` instances
- Add Markdown file fixtures for entrypoint
- Add `mock-require` package
- Add entrypoint test suite that mocks the `unified` plugin dependency
  with a spy
- Add specs for invoking the `unified` plugin's attacher and transformer

- Add `unist-diff` package
- Create a diff matcher for unist/mdast trees
- Add a spec for the unified plugin to test the overall functionality by
  comparing the modified input tree result to an expected output tree
- Fix issues with the expected output fixture

- Extract formatting of block objects (e.g. unist nodes) into a helper
  function
- Add helper for formatting inline objects onto a single line
- Move formatting into a separate helper module
- Update verbiage of a few error messages

Move context message to `withContext` calls.

- Fix output fixture
- Add spec to the entrypoint test suite that tests that the output tree
  provided by the unified plugin transformer is written to the given
  output path

Remark plugins should be standalone and leave file handling to
`remark-cli`. Removing the entrypoint and all associated specs and
dependencies makes way for `plugin.js` to be the main file for this
package, allowing to to be used with `remark-cli`.

Rename `plugin.js` and associated test files to use `index`.

Update the package name, repo, and description to reflect it being a
remark plugin.

Initial Implementation

Add CHANGELOG

- Configure `npm version` commit message
- Add scripts for automating release branch creation

Configure version bump

- Pull in more version script updates
- Add reference to script for `npm run mergeversion`

Update version scripts

Update CHANGELOG

Install Remark CLI