feat: replace with the changelog task from karma

The main thing I care about is the output, here's an example:
https://github.com/karma-runner/karma/blob/master/CHANGELOG.md

Couple of differences in the output:

- bigger header for major releases
- if a scope has only one change, it's on a single line, otherwise nested lists
- no scoped changes
- links to closed issues
- anchor/link to each release

Codewise:

- there are tests and travis build
- the code itself is grunt agnostic

Changes:
- the template is not configurable - I think having configurable template (as this plugin has now) is better, however during refactoring I realized that it makes it very hard to achieve the formatting I want (eg. different header for minor/patch release). Also, the template currently used is pretty much imperative JS anyway, so I went back to the original imperative style. I think we can allow people to configure individual template fragments, rather than the whole template (eg. minorHeaderTpl, patchHeaerTpl, changeItemTpl, etc...)

- **by default the output is written to CHANGELOG.md** - I think this is the 99% use case and therefore reasonable default; also this version does output some logging, which would clobber the actual output (however, the code is agnostic of writing into file, this is only done in the wrapper grunt task; so we can easily change this back)

- **adding editor option** - you can set this to say `sublime -w` and during the release process, you can amend the auto generated changelog before commiting

- **removing `enforce` option** - the git commit hook should not be installed during generating the
  changelog, we can add another task for it
chore: add travis config
chore: reformat the changelog

This is just re-generated by the updated task.
feat: parse Closes/Fixes from subject
fix: Write whether file exists or not
feat: parse multiple "Closes" definitions
chore: add watch task for running the tests
fix: append if prepend set to false

This just got lost during the refactoring.
chore: update readme
v1.0.0
feat(validator): show list of available types on error
fix: make changelog work if no githubRepo specified