No need to compile them once for matching on the url,
once per tag, once per title, once per author, … one time is enough.
It also simplify error handling, since while regexp compilation can fail,
matching can't.
If the user doesn't display reading times, there is no need to compute them.
This should speed things up a bit, since `whatlanggo.Detect` is abysmally slow.
The recent HTTP client refactor in 14e25ab9fe
introduced a bug in which the global default User-Agent is no longer
used for requests. Unless a per-feed User-Agent exists, the Go standard
library's default User-Agent is used, which looks something like
"Go-http-client/1.1". To fix this, make RequestBuilder.WithUserAgent
take an additional argument, the default User-Agent, which will be used
if there is no per-feed User-Agent (i.e. it is an empty string).
Fixes#2188Fixes#2189
