Added encoding optional parameter to subtitle file loading feature

[aalkz]

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• I have added a test to the test suite, if necessary
• I have properly documented new or changed features in the documention, or the docstrings
• I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

I had many issues with file encodings because I deal with special characters in my SRT files and this was a necessary step for me to solve them for good. It's an easy and straightforward change, but really handy if you need to specify your encoding when loading the SRT file.

Before you could only do this:
sub = SubtitlesClip("subtitles.srt", generator)

Now you can also specify your encoding if you want or need to:
sub = SubtitlesClip("subtitles.srt", generator, encoding='utf-8')

The encoding parameter is optional.

Special thanks to @Darkade for helping me figure this out.

[coveralls]

Coverage decreased (-0.02%) to 64.318% when pulling 3dbc6ee on aalkz:feature/subtitles_encoding into 3fc80cb on Zulko:master.

[tburrows13]

This is a neat feature. It took us a while to get to it, but I'm happy to merge now.