add -h option

[jessexknight]

I came all the way here just to learn how use the darn thing! Could you add a -h option for quick reference of the available flags? I did manage to forget that man exists, but evidently I've found -h more common in the tools that I use...

Many thanks,

If I have time and you're open to it, perhaps I'll add a PR.

[pkubowicz]

Currently my work on opendetex is limited to reviewing PRs and fixing most obvious bugs. I prefer that C code is written by people who have frequent contact with manual memory management, and I no longer belong to those people ;-)

If you have time, I will be happy to review your PR.