Syntax Highlight for overdue dates

[victal]

Adds support for #44 .

From what I tested even on a system without python the plugin continues working fine.

Maybe an option to supress the statup notice could be added if it turned out to be too annoying.

[freitass]

@victal this is a nice addition! I liked the message warning the user if his version of vim doesn't support python. However, I don't think the submodule approach to be the best solution. This plugin is supposed to be really simple to install and use. Asking the user (specially those who use plugin managers) to run git submodule update --init might drive them away. Would you consider moving that logic to this project?

[victal]

Good point. Actually vim-plug did handle the submodule correctly here but I was not sure about other managers, hence the warning.

Updated the PR moving the regex logic inside the plugin and removing the submodule.

[freitass]

Thanks!