If run with no arguments, `bst-here` now opens an interactive Bash
shell. This has several advantages to running each `bst` command
in a new container:

  * it's faster
  * bash completions will work
  * man pages could work (if the user runs `dnf install man`)

If run with arguments, those are now assumed to be arguments for `bst`
such that instead of running `bst-here bst build foo` you now just run
`bst-here build foo`.

This allows users to distinguish bst-here shell sessions from their
regular shell.

The prompt is blue and gives you just the path name, e.g.:

    /src>

The prompt can be customised or removed by editing the bst-here script
directly.