Add an option to use user/system hgrc files

By default, the plugin ignores user and system .hgrc files (e.g. `~/.hgrc`, `/etc/hgrc`, `%USERPROFILE%\Mercurial.ini`, etc.) by setting the `HGPLAIN` environment variable.

Occasionally, a user may want to use `/home/jenkins/.hgrc` to configure something the plugin doesn't have an explicit option for.  For example, I use it to remove compression from bundles when using Repository Caching as bzip2 compression is very slow.

This gives the user an option to use user and system .hgrc files at their own risk.  This is off by default and I added a caution message to the help file.

What do you think?