Finish flags and positional parameters

Closes #44 (closed)

This removes old positional parameters, automatically detects and loads subtitles and videos, and includes all necessary flags.

With this, the user can do the following:

• Open Subtitle Composer standalone. (Already possible)

• Double-click a subtitle to open with Subtitle Composer and have it load automatically. (Already possible)

• Right-click a video to open with Subtitle Composer and have it load automatically.

• Load video/subtitle with one parameter.

Example: subtitlecomposer video.mp4

• Load video/subtitle/translation with flags.

Example: subtitlecomposer --video video.mp4 --subtitle primarysubtitle.srt --translation translationsubtitle.srt