Make --export-latex optional.

This commit adds a latex=[true|false] key to the \includesvg command in
order to provide a mechanism for skipping the --export-latex feature
from inkscape. If no value is specified, the feature is turned on.

References #1.
Add a package option to switch between latex/pdf codegen.

References #1.
Forward all unknown options to \includegraphics.

References #1.
Merge pull request #2 from pitti/feature/optional-latex-codegen

Feature/optional latex codegen