ENH: specify output file, or print to stdout

This change involved modifying converters which define their own
`__init__` methods to accept arbitrary keyword arguments (kw) and pass
them on to the call for the ConverterBase (which will work nicely when
we add other arguments to the base converter).

To convert to `notebook.ipynb` using the default converter, but send the
output to a new file called `foo.bar`, use:

    nbconvert -o foo.bar notebook.ipynb
    nbconvert --outfile foo.bar notebook.ipynb

To send the converted output to standard out (so it can be visual
inspected, or piped, etc), use:

    nbconvert -O notebook.ipynb
    nbconvert --stdout notebook.ipynb