fix parsing of user-provided command line arguments

Matlab is actually quite good about keeping user-provided arguments together. See:

>> dynare example1 '-DA="nthn   thnth"' -DB=[1,2, .3] -DB=(    oe )
-DA="nthn   thnth"
-DB=[1,2, .3]
-DB=(    oe )

The output above comes from simply printing the arguments of varargin.

We should be able to thus simply add single quotes around every user-provided argument in dynare.m and pass these directly to the preprocessor for processing. We'd then require that all strings be double-quoted and could potentially even handle macro expressions (not just base values) in -D arguments.