stbt control: Allow specifying keymap file in stbt config

With this change the keymap file will default to the value stored in
"control.keymap".
Merge branch 'config-control-conf' into next
WIP: Region: Add extend method

TODO:
* Write tests
* Write documentation
WIP: Report: Strip HTML from notes in table

Come up with a better idea.
_numpy_from_sample: Improve error reporting

Print a more useful message when wrong type passed to `_numpy_from_sample`.

I ran into this when mistakenly passing a region as the first argument to
`stbt.ocr`.
WIP: Add "next" and "prev" links to debug HTML pages

TODO:
* Write tests
* Make it look better by right aligning next and left aligning prev
fixup! virtual-stb: Add virtual stb - useful for testing virtualized STBs
WIP: HTML in notes can use the failure and success classes
fixup! virtual-stb: Add virtual stb - useful for testing virtualized STBs