- don't expose the evbuffer where we store the cached page - cache to/from tabs instead of buffers

the idiom if (load_next_page(...)) start_loading_anim(...); is fundamentally wrong because `load_next_page' (or previous page, or anything else that call into load_url) *may* load all the page before returning (thanks to the cache, but not only, cf. load_page_from_str.) Instead, let's call start_loadign_anim at the start of make_request so it's always called in the right moment. Cache and other special pages don't end up calling make_request anyway, and we already call stop_loading_anim when a request finishes (or fails.) move start_loading_anim up in make_request.

This changes the last argument of load_url to be a `mode' bitmap instead of a boolean ``nohist''. LU_MODE_NOHIST is the old 1, LU_MODE_NONE is provided just for readability and the new LU_MODE_NOCACHE allows to bypass the caching when loading a page. Telescope only uses the cache when: - re-opens a closed tab - navigates backward and forward in the history All other situations result in the usual network request.

see github issue #1

print_vline can handle a NULL .line, so I can avoid the long comment and the ugly cast.