SUPPORTED_WRITERS to parallel SUPPORTED_READERS

[acdha]

I wanted a dictionary similar to SUPPORTED_READERS to make it simple to have a conversion tool a text name to lookup the writer class to use — e.g. --output-format=webvtt. This is obviously pretty easy to handle but it means I have a hard-coded list which would be nice to avoid:

https://gist.github.com/acdha/c9fd54d4dee67801a09b#file-convert-subtitles-py-L24-L30

[ianShifrin]

We currently don't have plans to add new writers so a hard-coded list is probably ok to use. Thanks.