Resolved issue #546. Output format parsing from filename extension.
Added back `-t` option. If `--output-format` not defined parse from extension `--output`
Fix default value.
Add import os for crawl.
Added more verbose error message for unrecognized output format. PEP8.
Merge pull request #659 from denysbutenko/master

[MRG] Resolved issue #546. Output format parsing from filename extension.