Add support for on-demand subtitles

[JonnyWong16]

Description

Changes:

• New methods Video.searchSubtitles to search for on-demand subtitles and Video.downloadSubtitltes to download on-demand subtitles.
• Cleaned up uploadSubtitle and removeSubtitles methods and updated doc strings.

Breaking change:

• Added subtitleStream as first argument to Video.removeSubtitles.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable

[blacktwin]

Flipping through the hearingImpaired and forced value combinations while searching for subtitles and SubtitleStream() should probably get additional attributes for hearingImpaired and perfectMatch. Both seem to either not exist or exist with the value of 1.

[JonnyWong16]

Added the hearingImpaired attribute (bool) which will always be True / False. This matches the forced attribute.

Added the perfectMatch attribute (bool) which will be None / True. I did not want this flag to be False because that can be misleading for subtitles that are not downloaded on-demand.

[glensc]

downloadSubitltes typo