There has been a desire from more advanced rclone users to have regexp
filtering as well as the glob filtering.
This patch adds regexp filtering using this syntax `{{ regexp }}`
which is currently a syntax error, so is backwards compatibile.
This means regexps can be used everywhere globs can be used, and that
they also can be mixed with globs in the same pattern, eg `*.{{jpe?g}}`
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
