Support custom config file name and publishdir

[silsha]

This pull request adds an option to use a different config file name and/or publishdir.

You can set your heroku environment variables with

$ heroku config:set HUGOCONFIG="config_de.toml"

and

$ heroku config:set HUGOPATH="public/custom"

[elsbrock]

Initialize $CONFIG_FILE to an empty string, then remove this branch entirely.

[roperzh]

Hello @silsha!

Thanks so much for such a useful pull request! I'm very sorry for the delay, but I have been really busy.

I really like your approach, and I'm wondering why I decided to use a .hugotheme file instead of an ENV variable (ツ)/¯.

Anyway, what are your thougts about using a more general approach to set hugo flags? Instead of having HUGOCONFIG we can have a HUGOFLAGS variable:

HUGOFLAGS="--config='config_de.toml'"