Add command line flag to load level directly

[SalsaGal]

Using the -i [FILEPATH] argument you can choose a level to load directly on game launch. Fixes https://github.com/open-manifold/Open-Manifold/issues/7.

[SuperFromND]

Oh wow, great work! I did notice a couple things testing this PR though:

• The menu music commit (https://github.com/open-manifold/Open-Manifold/pull/23/commits/f85cba70fc01c7ffd63a5b7956a99a2ed8006b1b) now results in the main menu music restarting if you exit any other menu (options, level select, etc.); this was my initial reason for putting the load command at the end of WARNING (forgot to comment that being the case, my bad!) A rather minor issue, and one I'm willing to overlook for now since it probably doesn't bother anyone besides myself. :P

• Loading the level here requires specifying a folder, not a level.json file itself (and in fact will actually crash if you try to load a .json file). This might be confusing from a UX perspective; I would suggest tweaking the help message to specify a 'folder/path' rather than a 'file name', or even using a dummy example:

  "-i [folder/path] - Load a level on start\n"); AND/OR... "-i P:/ath/to/lvl - Load a level on start\n");

  • Or alternatively, figuring out a way to detect and strip a given input string of a specified file, giving us just its parent folder (so e.g. stripping Z:/Path/To/Level/level.json to just Z:/Path/To/Level/); a regex like ([^\/]*)$could probably help with that (it matches with the level.json part).

[SalsaGal]

Ah ight I'll try getting these sorted out and let you know how it goes

• [x] Fix menu music: Fixed with https://github.com/SalsaGal/Open-Manifold/commit/45fbbbe28622a5a6fbea7613347996c695c6ee3a, was as simple as just undoing the commit because I had fixed the problem as a side effect of another commit.
• [x] Load level with json or clarify message
• [x] Detect and strip level path

[SalsaGal]

Alright that should be everything done!