Accept input from STDIN as well as a file

[roryokane]

I would like to be able to do pbpaste | html2slim | pbcopy to process the contents of the clipboard.

Current usage:

Usage: html2slim INPUT_FILENAME_OR_DIRECTORY [OUTPUT_FILENAME_OR_DIRECTORY] [options]

Desired usage:

Usage: html2slim [INPUT_FILENAME_OR_DIRECTORY [OUTPUT_FILENAME_OR_DIRECTORY]] [options]

erb2slim should be changed similarly, of course.

[tlindsay]

This is throwing a segfault for me. This exact use case is the only reason I even have to use html2slim, so +1.

[joaomilho]

Could be easily done in a PR, any takers?