Feature: --watch should add a date for last updated

[rreilly-edr]

with the watch option it would be great to see the last date time it was updated or the mtime of the file. just a thought, this is a great tool.

[lukejpreston]

we used to have this, but then old impl kept breaking between linux and macs. Do you mean instead of output.html to have output.${time}.html (for example)? I have a solution which shouldn't break both OSs

[lukejpreston]

I think to solve this I need to add --timestamp as an arg which will automatically add the time to the file e.g. output-name.timestamp.html and the arg could be a date format