vterm.py: Adds support for bracketed paste mode. Fixes #452

[danschwarz]

NOTE, this is a new PR replacing #576 . All the same code, plus automated tests included now.
Please refer to the #576 comment thread for discussion of what's going on here and why.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Note that bracketed paste mode is NOT automatically enabled for applications; this would cause problems for any apps that aren't bracketed-paste-aware.

There's an example of how to enable it in your urwid application in examples/terminal.py.

Then you need to actually handle the 'paste begin' / 'paste end' key sequences in your application's keypress method. There's no example code for this yet.

For more info on bracketed paste mode, see Wikipedia
or https://cirw.in/blog/bracketed-paste

[coveralls]

coverage: 73.458% (+0.3%) from 73.188% when pulling bced054 on danschwarz:bracketed2 into 2e3a7ec on urwid:master.