Cache URLFilter results of string inputs per site

[ashmaroli]

Summary

Currently on master, the results for relative_url and absolute_url are cached per Liquid:Context instances. But this is insufficient for string inputs which are context-independent.

This PR proposes to cache such results per Jekyll::Site instance.

[ashmaroli]

Comparisons

Verbose build with --profile summary

--- master https://travis-ci.org/jekyll/jekyll/jobs/649458789
+++ PR     https://travis-ci.org/jekyll/jekyll/jobs/649466901

$ bundle exec jekyll build -s docs --verbose --profile

- done in 6.351 seconds.
+ done in 5.358 seconds.

Memory Profile summary

--- master https://travis-ci.org/jekyll/jekyll/jobs/649458790
+++ PR     https://travis-ci.org/jekyll/jekyll/jobs/649466902

- Total allocated: 396.91 MB (3624650 objects)
+ Total allocated: 384.73 MB (3490922 objects)
- Total retained:  19.80 MB (107264 objects)
+ Total retained:  19.74 MB (106858 objects)

[ashmaroli]

Memory Profile summary

--- master https://travis-ci.org/github/jekyll/jekyll/jobs/669670236
+++ PR     https://travis-ci.org/github/jekyll/jekyll/jobs/669699899

- Total allocated: 370.27 MB (3400304 objects)
+ Total allocated: 348.19 MB (3168258 objects)
- Total retained:  19.09 MB (108407 objects)
+ Total retained:  18.97 MB (107431 objects)

/cc @DirtyF 😃

[DirtyF]

Cool, it's working great.

[DirtyF]

@jekyllbot: merge +fix