Implement conditional actions

[pgaskin]

Closes #11.

[pgaskin]

I've tested this with:

menu_item:main:Test:cmd_output:500:mkdir /tmp/geek1011 && echo mkdir
chain:dbg_msg:Success
chain_failure:cmd_output:500:rm -rf /tmp/geek1011 && echo rm
chain_always:dbg_toast:Done

I've also tried it in a few actual ones to make sure it works with more complex chains

[pgaskin]

Also, @baskerville has tested this before I opened this PR.