Add --cmds option for passing commands

[alexvh]

pdflatex and xelatex accept either a file or commands. Add similar functionality for pydflatex with --cmds option.

From commit:
Similar to passing commands instead of a filename to pdflatex or
xelatex, but still allows for pydflatex to do any path magic and
output file finding it wants. Also allows for using commands with
latexmk, which normally only takes files (e.g. latexmk
-pdflatex="pydflatex --cmds '\def\webver{}').

e.g. Passing parameters to the tex file
Tex file:
\ifdefined\webver
web version
\else
normal version
\fi

pydflatex --cmds '\def\webver{}' texfile

[olivierverdier]

Nice! Great idea. Let me look at this.