allow processing multiple files at once

You can now pass multiple files (e.g. subberthehut *.mkv)
and subberthehut will download subtitles for each of them. subberthehut
will exit as soon as it fails to download a subtitle for one of them.
Add option to not exit when failing one of multiple files

By default, subberthehut will exit if one of multiple passed files fail
to download a matching subtitle. If this option is passed, subberthehut
will continue processing the next files regardless.
dont pass lang to functions as parameter

Like all other options, this is a global parameter and it is pointless
to pass it around as a function parameter.
fix possible trailing '/' on filename

If the file was not in the working directory, the filename
included the '/' that strrchr pointed us to.
limit to 10 results by default / allow specifiying limit

You can now specify the maximum number of results returned
by the query. This defaults to 10.