Add command line option for choosing builder

[rafaelnp]

Add command line option, to enable the user to choose which builder he/she wants to use. Suggestion:
--builder BUILDER specify the builded to be used

By adding it, it would allow:

1. Users to actively choose the builder the way to use, without any code change (e.g. editing hdl_checker/builder_utils.py).
2. to separate use cases and make it easier to track bugs that may arise.

[SethGower]

I know this is way late, and I am not @suoto, but figured I'd respond since I am looking around and trying to contribute. This isn't documented, but you can use the tag in the config file "builder" to specify which builder to use. This was added here. I have tested it with ghdl vs msim and it seems to work. The available options are msim (for Modelsim), xvhdl (for Xilinx Vivado), and ghdl (obviously, for GHDL). These can be seen here