Adds support for POSIX shell startup file

The 3 are ordered in ascending fanciness.
Updated README

I normalised use of the terms utility/tool/command to command, similar
for `pro`, and emphasised that `pd` is a shell function (At least to
me, it previously wasn't clear what it was and why one command would
install another).
Merge pull request #4 from frou/pro-install-with-posix-shell

Made `pro install` work for sh users