Add Language.find_by_extension
Add some tests for Language.find_by_extension
Fix docs
Allow extensions without a leading dot

This makes the API a little easier to use.
Merge pull request #1683 from github/find-by-ext

Extract Language.find_by_extension
Bumping to 3.5.0
Merge pull request #1694 from github/cut-release-v3.5.0

Bumping to 3.5.0
Make it safe to pass nil to Language.find_by_name/alias again

This restores compatibility with v3.4.x.
Merge pull request #1695 from github/nil-safety

Make it safe to pass nil to Language.find_by_name/alias again
Bump to 3.5.1
Merge pull request #1696 from github/cut-release-v3.5.1

Bump to 3.5.1
Add F# sample.  Add Forth extension .fs.