[WIP] Allow for selecting test case by line number

[blueyed]

This parses the line number from foo.vader:123 and executes only the
case ("Do" or "Execute") around this line number.

Any previous Given/Before/After/Then is copied (like s:parse_vader would
do it).

TODO:

• after Display source with exceptions #107

[junegunn]

Thanks. vader#run is already too long, and I'd like to trim it down a bit before adding more lines to it. Can you rebase your work onto the following commit? (I didn't merge it to master branch)

2581d00

Any previous Given/Before/After/Then is copied (like s:parse_vader would do it).

A Then block is supposed come after Execute or Do, do we need it here?

[blueyed]

Ok, will do it.
I've noticed that it does not work as-is anyway, since it uses the fpos additions from #107.