Preserve _ prefixed to variable names when generating erlang

Completely backwards compatible, but means we can enable
warnings_as_errors, and name unused variables with an underscore.
(see 2 new tests)
Merge branch 'unused-vars-fix' of https://github.com/irccloud/erlydtl into irccloud-unused-vars-fix
rename varify/1 -> varname_ast/1