Add flag for format and option to compile LaTeX

A new set of flags for the output format was created. Based in
this flag the backend change it behaviour.
Fix problem when not using --pdf or --tex

Thanks to @abelsiqueira to report the bug.

To fix the bug `output_format` has `None` as the default value.

Also make that `--pdf` "require" `--tikz-header`.
Fix bug related with pdflatex output directory

As report by @abelsiqueira the PDF was created in the current
directory and not in the output one.
Merge pull request #29 from r-gaia-cs/output-format

Add flag for format and option to compile LaTeX