3.1 support

[stevie-mayhew]

There is already the great work of @timsnadden here https://github.com/timsnadden/silverstripe-customhtmleditorfield

Do you think you would be able to cherry pick that across to this repo? I've also emailed @timsnadden in case he can generate a PR 😄

[timsnadden]

I've create a pull request for using yaml config #12

That being said, I believe that this repo should work in Silverstripe 3.1 as it stands. The differences from my fork are just whitespace and fixes to comment typos (apart from the changes in #12).

[nathancox]

Yeah it does work on 3.1 as-is but apparently I need to get better at remembering to tidy my code before I put it on Github. Anyway I accepted the pull request. Thanks for the interest everyone.

[stevie-mayhew]

Cheers @nathancox and @timsnadden - excellent stuff.