Add "implements" to the default issue closing commit message regex

Description

I generally use "fixes #x" in the commit message for commits that fix bugs and "implements #x" for commits that implement features. The latter is not detected in Gitlab, and this feels like asymmetry.

Proposal

I propose that the syntax "implements #x" be added to the issue closing regex.

Links / references

https://github.com/gitlabhq/gitlabhq/issues/302

Documentation blurb

This translates to the following keywords:

Close, Closes, Closed, Closing, close, closes, closed, closing
Fix, Fixes, Fixed, Fixing, fix, fixes, fixed, fixing
Resolve, Resolves, Resolved, Resolving, resolve, resolves, resolved, resolving
Implement, Implements, Implemented, Implementing, implement, implements, implemented, implementing

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml