Export to a specific file

[joeyespo]

The --export option allows exporting Readme files to a single HTML page. Currently, it uses the filename of the input file, but with an .html extension. This should be extended to specify an output file.

Some thoughts

1. When --export is used, the server isn't run, so the optional <address> argument will be unused. We could have the second argument specify a directory or file to write to in this case.
2. We could also just have an optional --export=<filename> variation if docopt supports it.

[joeyespo]

Opting for (1), since (2) is ambiguous / not allowed in docopt.

[joeyespo]

Fixed in acb0cd04b5ef028188945705982b02940457a5d3.