feat: allow fixup! and squash!

[cmalard]

fixup! and squash! are part of Git, commits tagged with them are not intended to be merged :

• http://fle.github.io/git-tip-keep-your-branch-clean-with-fixup-and-autosquash.html
• https://git-scm.com/docs/git-commit

$ (dev) git commit -m "Feature A is done"
[dev fb2f677] Feature A is done

$ (dev) git commit -m "Feature B is done"
[dev 733e2ff] Feature B is done

# you've removed a pdb : shameful commit
$ (dev) git commit --fixup fb2f677
[dev c5069d5] fixup! Feature A is done

$ (dev) git log --oneline
c5069d5 fixup! Feature A is done
733e2ff Feature B is done
fb2f677 Feature A is done
ac5db87 Previous commit

$ (dev) git rebase -i --autosquash ac5db87
ff4de2a Feature B is done
5478cee Feature A is done
ac5db87 Previous commit

If we do not want to allow fixup/squash (this is not in the Angular commit spec, after all), the way to use this feature is to ignore validation on the squash/fixup commit (git commit --fixup --no-verify). I am not in favor to make this a habit, hence the PR :-)

[codecov-io]

Current coverage is 100.00%

Merging #19 into master will not affect coverage as of 681df95

@@            master     #19   diff @@
======================================
  Files            1       1       
  Stmts           53      54     +1
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit             53      54     +1
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 681df95

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[kentcdodds]

Awesome. Thanks!