Bugfix: dimensions not actually returned
Guard against empty command string
Allow VtrSendCommandToRunner cmd to accept args
Create runner pane in temp window

In order to avoid rapid flickering of runner window as it is created
and initialized, create and initialize it in a temporary background
window then bring it in.
Change default to not clear on resize
Accept optional new_percent arg to Resize command
Add SendSelected and SendLine commands

These commands allow for arbitrary text in the Vim buffer
to be passed to the runner pane for execution.