use dynamically initialized var for plugin dir

[jirgn]

Using the specific path $ZSH_CUSTOM/plugins/flow does break compatiblity with https://getantibody.github.io/.
This pluginmanager creates a folder underneath ~/Library in the form ~/Library/Caches/antibody/https-COLON--SLASH--SLASH-github.com-SLASH-sandstorm-SLASH-oh-my-zsh-flow-plugin.

The Failing behavour looked exactly like #10 and #16

flow (eval):1: command not found: Could

To address this issue i created a global var which is initialized on plugin load with the relative path to the plugin file ($0).
I hope this also fixes the other issues

[skurfuerst]

thank you :)