feat(conventional-changelog): adds grunt-conventional-changelog task

this adds brian's awesome grunt task to generate changelogs automatically
through well-formed commit messages.

there's now a new section in the gruntFile.js called 'changelog', which
specifies the options for the task.

for more information about the grunt plugin checkout btford/grunt-conventional-changelog
its readme file also describes where you can find the specification for
well-formed commit messages.

when running [4mRunning "changelog:FROM_REF" (changelog) task[24m
fatal: ambiguous argument 'FROM_REF..HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions
[33mWarning: Task "changelog:FROM_REF" failed. Use --force to continue.[39m

[31mAborted due to warnings.[39m make sure  is an existing tag in your repository.
Merge pull request #31 from PascalPrecht/conventional-changelog

feat(conventional-changelog): adds grunt-conventional-changelog task