Command line parameters for search paths

[Bohun9]

This commit should resolve #85.

I am not sure if it is possible have functionality of specifying multiple arguments to a single option, like dbl -I path1 path2 source.dbl with Arg API easily.

[forell]

I'd like to suggest some small changes before merging this pull request. I think you should swap the -I and -L options so that -I configures local_search_dirs and -L configures lib_search_dirs. While the search paths don't accurately correspond to the traditional flags from C-related infrastructure, for now I believe this way will be more intuitive, unless you can convince me otherwise.

By the way, using -I or -L multiple times for every directory seems fine, and space-separated arguments to these options would clash with how all anonymous arguments after the filename are treated as arguments for the interpreted program anyway. So, I think this part is good as is.

[forell]

Expressions like xs @ [x] performed in a loop tend to look a bit fishy (though, to be fair, these lists are unlikely to be long). Besides, I'm not sure that we actually want to add directories from command-line options at the very end; perhaps they should take precedence over some other potential sources (e.g., environment variables), even if presently it makes no difference. I think you can solve both of these minor issues by accumulating these directories in reverse order into a temporary list, and List.rev_appending them with the list in Config after parsing all arguments.

Also, this line is too long.

[forell]

This option takes just one path, which I don't think is clear from the description. I'd also prefer it to be phrased in terms of an action that the option will perform, like "add a directory ...", to be consistent with the other help messages.

[forell]

Same feedback as above.

[Bohun9]

I agree that swapping the meaning of the flags is a good idea. I added changes you suggested.

[forell]

Looks good, thanks!