Add pick-exec to list of useful commands
Export sensible defaults in pick-exec, fixes #1

* $DISPLAY defaults to `:0`
* $GPU_MAX_ALLOC_PERCENT defaults to `100`
* $GPU_USE_SYNC_OBJECTS defaults to `1`
Fix pick-update to allow branch changes
Fix pick-use to allow full version specifiers
Document pick-update in readme
Bump version to 0.1.1 and document changes