Preserve _ prefixed to variable names when generating erlang

[RJ]

Completely backwards compatible, but means we can enable
warnings_as_errors, and name unused variables with an underscore.

Otherwise it's impossible to use warnings_as_errors if you need to for loop over a proplist with keys/vals and you only want the vals. (for example).

See 2 new tests for the details

[RJ]

Other options include:

1. always prefix all variables with _ in the erlydtl_beam_compiler

2. passing nowarn_unused_vars to the erlang compiler, possibly via compiler_options?

[kaos]

Related discussion: https://www.irccloud.com/pastebin/zjXRdvty

[kaos]

👍 Looks good. I'll just touch up the name varify a bit..

I prefer this instead of messing around with compiler options. nowarn_unused_vars can be passed using compiler_options already, and can be a workable option for those that don't like to use _ prefixes in their templates.