YAML file recognition for Inform 7, an interactive fiction design language.

The team account https://github.com/i7 has several active Inform 7 projects.
Inform has long lines, so we line wrap.
Merge branch 'master' of https://github.com/github/linguist
Added a lexer definition (text only).
I may build an Inform 7 highlighter/lexer someday, but not this week.
Merge pull request #897 from erkyrath/master

YAML file recognition for Inform 7