rewrite: add 'bn_' banner only rewrite
cleanup rewrite_content/fetch_request api to take a full wb_url
add content-length to responses whenever possible (WbResponse) and static files
bump version to 0.5.2
proxy resolver: cookie resolver uses session cookies
proxy static handler: handled via proxy to support http/https
use 'pywb.proxy' prefix for custom env settings
Merge branch 'develop' into https-proxy for 'bn_' modifier support
proxy mode: cookie based selector using session to coll
ui: add proxy_selector html, add switch link to error and banner
proxy mode: make banner replay and cookie resolver configurable via proxy_options