rename purge_tags_header config to cache_tags_header

[dbu]

great job, really like this!

looking over the thing, i noticed that i would prefer this name for the config. the meaning of the header is cache tags, not purging.

also added a bit more explanation for the tagging.

a note on the default header name: these days, the recommendation is to omit the X- from custom headers. but for FOSHttpCache we in the end decided its too much hassle for too little benefit to refactor everything to drop them. this means that the default value will not work, because we still use X-Cache-Tags. but i think we can handle this in the doc. its better if you have a clean thing here in this new library.