Command-line tweaks and folder structure

[ocornut]

(I'm splitting this away from #1 so we can track smaller chunks.)

• [x] Change the convert_header() call to pull input filename, output filename and template from command-line parameter. Provide a batch/sh file to do the most common thing for cimgui/cimgui_internal. Maket he program display basic instructions if ran without parameters.
• [x] Move most of the code in a sub-directory, leaving root directory with main.py (maybe rename) + licence/changelog + 1-2 directory (maybe we can include an "output/" directory in the repo but that requires a dummy file). This is mostly to make using the whole thing incredibly obvious at first glance, hide the guts away.
• [x] Return error code so it can integrated in CI (we can help setup add github actions CI to run this over master+docking if you like?)

[ShironekoBen]

I've updated the code to be much more normal-program like and actually have a command line interface now, as well as hiding away the internals in a subdirectory. Error codes are now also a thing so if CI seems useful we should be able to do that (we probably need a more extensive test project to allow CI to check that the results compile properly, TBH, but arguably even just having CI check the results against the last ones and email out a diff could be interesting as a way of keeping track of if any changes actually look like they might have broken something).

[ocornut]

Thanks!

I moved the documentation files into a docs/ subfolder as github allows that (still shows readme from web). Amazing work on the readme!

@rokups has great experience with github CI so we'll add some form of CI (running dear bindings + compiling/calling result).