Handle shebang more thoroughly in scripts plugin

[canadaduane]

This PR allows the shebang line in a script file to have more flexibility:

1. Whitespace is accepted after the #! characters, so #! /bin/bash is now ok.
2. Any args passed to the command in the shebang are honored, so #!/usr/bin/env nu will work (for example)

Fixes #142 (except for stderr and non-# metadata comment lines)