Add option to print SimpleMRS with color.
Allow MRS codecs to ignore extra keyword arguments.

This prevents MRX and DMRX from crashing with the previously introduced
color arguments for SimpleMrs.

This commit also allows the top-level mrs.py script to accept STDIN as
input for MRSes, instead of just files.
MRS codecs now operate on lists of MRSs.

The load(s) and dump(s) of DMRX, MRX, and SimpleMrs now read and write
lists of MRSs instead of single MRSs. This also means that MRX and DMRX
use the <mrs-list> and <dmrs-list> root nodes, respectively.