Add focus (f) and pending (x) prefixes support
Add capybara’s ‘background’ keyword

`background` is an alias for `before`.
Add choices to the `before` snippet
Add the missing `let!` snippet
Add missing capybara snippets

- background (back⇥)
- given (given⇥)
- given! (given!⇥)
Merge pull request #95 from elia/x-f-prefixes-syntax

Add/improve some keywords and snippets.