Add cucumber checker for cucumber files

For now, it only detects undefined steps. The nearest `features` dir
above the buffer file is loaded, so step definitions should be found
correctly.

Tested only with Cucumber for Ruby, but it should work for any cucumber
that follows a substantially similar directory structure.

doc/ale.txt

@@ -322,6 +322,7 @@ Notes:
 * CoffeeScript: `coffee`, `coffeelint`
 * Crystal: `crystal`!!
 * CSS: `csslint`, `prettier`, `stylelint`
+* Cucumber: `cucumber`
 * Cython (pyrex filetype): `cython`
 * D: `dmd`
 * Dafny: `dafny`!!