Displaying Op command line in UI

This implements #793.

On a related question, I'm finding myself wanting to be using Markdown when writing description/summary Metadata for nodes and plugs, and for tooltips in general. Currently we're using a little bit of HTML here and there in tooltips, which is useful but a bit ugly, and not suited for general Metadata where we do things like generate PDFs from it. What does everyone think to making Markdown our format of choice for this stuff? Would the basics work in Asciidoc as well @bentoogood for converting Metadata to full docs or are the syntaxes quite different?

There seem to be a couple of implementations of Markdown for Python out there already :

https://pypi.python.org/pypi/Markdown
https://github.com/trentm/python-markdown2

I'm a bit loathe to add more dependencies but I think this one could really improve the readability of documentation and tooltips, so it's worth considering. Since it's a small dependency that presumably doesn't change much, we could even copy it into our own source (the second link in particular is implemented entirely in one file, so would transfer very nicely).