Handle a -trace parameter for setting the trace level

A similar parameter already exists in the other version of
the ParseCommandLine function.

The WelsStderrSetTraceLevel isn't one of the functions that
is exported from wels.dll (nor welsenc.dll) though, so this
doesn't work (not currently either since the function is
currently already referenced) if we would try to link to the
encoder library dynamically.
Merge pull request #339 from mstorsjo/console-trace-param

Handle a -trace parameter for setting the trace level