Add --browser argument to nbdiff/nbmerge to specify browser to launch.

Fixes #113
Remove save button in nbdiff

Fixes #106.
Merge pull request #123 from tarmstrong/nbdiff-no-save-button

Remove save button in nbdiff
Merge pull request #122 from tarmstrong/browser-argument

Add --browser argument to nbdiff/nbmerge to specify browser to launch.