Format-specific initialization

Pycaption API right now is somewhat inconsistent in that `SCCWriter(some_arg="some val")` works but `SRTWriter(some_arg="some val")` results in exception since SRTWriter does not have an initialization method. Insofar as we can consider individual reader / writer as plugin and pycaption as the framework, I think it is important that pycaption exposes a consistent interface to the end-user.

3ace98c ensures that regardless of the writer, initialization of the form `WriterName(some_arg="some val")` will work. If a writer needs initialization, it can overload `__init__` from `BaseWriter`. If it takes any configuration parameter, user can pass it during instantiation. 0800dee provides an example use case of this feature.

You can read about WebVTT metadata header [here](http://dev.w3.org/html5/webvtt/#dfn-webvtt-metadata-header).