Install script -- allow specifying a target directory

[iandol]

Currently https://zyedidia.github.io/eget.sh downloads to the current directory. It would be a little less friction if we could optionally select a target directory eget could be moved to. Currently we have to do it manually and this is a bit more friction when trying to bootstrap a system. Thanks for such a useful tool!

Something like:

curl https://zyedidia.github.io/eget.sh | sh -s -- '/usr/local/bin'