add 'local install' and 'local upgrade'

[tsloughter]

This is a proposal for making it easy for a user to use rebar3 without the escript. Not relying on the escript allows for a faster start time and fully functioning shell.

To use:

$ ./bootstrap
$ ./rebar3 local install
$ export PATH=$PATH:~/.cache/rebar3/bin/rebar3
$ rebar3 <task>

[ferd]

Would it be possible to make this work with any escript archive? Sounds like it would be more generic, or even moved under that command. Also wondering how easy it would be to move things to windows. I can do work on this too.

[tsloughter]

I don't think we'd want it to work for just any escript. And to create the run script we need to know the name of the module to run and it has to use init:get_plain_arguments() for arguments.

Windows should be pretty simple to support.