Add support for custom ssh keys

[dignifiedquire]

Due to how cargo uses ssh keys, it is required to run

eval $(ssh-agent -s)
ssh-add $HOME/.ssh/my_key

before executing the install step, if there are any keys present that need to be used. There are a couple of build packs for heroku that allow to setup the key, but adding those lines above to this buildpack are required to make it work as I found out.

[emk]

Interesting! I would definitely be interested in a PR adding this feature, particularly if it had a link to existing Heroku build pack documentation, or to other major build packs including this step. Thank you for suggesting this!

[emk]

This is almost ready to merge.