Add load command

[ryansch]

Adds the ability to load a desk into the current shell. Useful for things like tmux where we've already started a new shell.

Usage: eval $(desk load <name|dir>)

Fixes #36

[megalithic]

@jamesob bump, pls

[megalithic]

@jamesob bump, pls; this is brilliant.

[jamesob]

Hey, sorry for the egregious delay here and thank you for the reminder. Looks good, thanks!

[jamesob]

The included test fails for me when I run it locally with make bash:

desktester@1b19b608cca5:~$ bash run_tests.sh
Desk dir already exists at /home/desktester/.desk
mkdir: cannot create directory '/home/desktester/terraform-repo': File exists
~/example-project ~
~
~/example-project ~
/bin/bash: line 0: export: `${DESK_ENV}': not a valid identifier
grep: "/home/desktester/example-project/Deskfile";: No such file or directory
grep: "/home/desktester/example-project/Deskfile";: No such file or directory
Failed: Deskfile load didn't work (./)

[ryansch]

@jamesob Try now!

[ryansch]

I also tossed in some Dockerfile updates.