Implement request variants handling

[chesio]

Motivation: every page on sites, where Cookie Notice plugin is active, has two variants: one with cookie notice banner and one without it. This way, only part of visitors can be served from cache: either those who haven't accepted cookie policy yet or those who did.

Idea: store alternative page variants (HTML responses) in directories with #variant-name appended to the directory name. This should be safe, as # will never be part of request URI (and thus default directory name), but is a perfectly legal character in directory name under both Unix/Linux and Windows.

[chesio]

Note: The feature would require URI to directory relationship to change from 1:1 to 1:N (one URI can be cached in multiple directories). Not a big problem now, but should be considered, because #4 might be implemented one day and cache API supports deletion of single URI already.

[chesio]

The final implementation did not include variant name in directory names, but in file names. It seemed more clean and was quite straight-forward to implement as the implementation aligns well with the handling of gzipped variants. Also, the 1:1 relationship between request URI and directory is retained.