[eruby] Add erubi linter

Erubi is yet another parser for eRuby. This is the default parser in
Rails as of version 5.1. It supports some additional syntax with similar
behavior to Rails' extensions to the language, though incompatible.
Rails currently still recommends their own syntax, so GetCommand still
has to do the translation introduced in
https://github.com/w0rp/ale/pull/1114 .

Erubi does not supply an executable—It is intended to be invoked only
from within a Ruby program. In this case, a one-liner on the command
line.

doc/ale-eruby.txt

@@ -1,17 +1,15 @@
 ===============================================================================
 ALE Eruby Integration                                       *ale-eruby-options*
 
-There are two linters for `eruby` files:
+There are three linters for `eruby` files:
 
 - `erb`
 - `erubis`
+- `erubi`
 
-If you don't know which one your project uses, it's probably `erb`.
-To selectively enable one or the other, see |g:ale_linters|.
-
-(Note that ALE already disables linters if the executable for that linter is
-not found; thus, there's probably no need to disable one of these if you're
-using the other one.)
+`erb` is in the Ruby standard library and is mostly universal. `erubis` is the
+default parser in Rails between 3.0 and 5.1. `erubi` is the default in Rails
+5.1 and later. To selectively enable a subset, see |g:ale_linters|.
 
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -286,7 +286,7 @@ Notes:
 * Dockerfile: `hadolint`
 * Elixir: `credo`, `dogma`!!
 * Elm: `elm-format, elm-make`
-* Erb: `erb`, `erubis`
+* Erb: `erb`, `erubi`, `erubis`
 * Erlang: `erlc`, `SyntaxErl`
 * Fortran: `gcc`
 * FusionScript: `fusion-lint`