Cache check whether library URL exists slow down build

[jannschu]

Hi,

first, thanks for this plugin!

I noticed that my total build times increased significantly after adding this plugin (went from 5s to 20s). Upon further inspection I noticed that the code checks whether the JavaScript library exists at the configured URL by sending a HTTP request on every page although the library URL does not change.

Caching the URL check did show a very nice performance boost and brought build time down to 6s, so I propose to implement caching for this.