Terminal logging is now coloured only on an interactive terminal

The terminal logger always coloured its text, so output piped or
redirected to a file, and the systemd journal, got escape codes. It
now colours only an interactive terminal.

The service's logs can therefore go to the journal: logging.file is
now optional for it, and the unit's comments and the README say so.

README.md

@@ -44,8 +44,10 @@ The `sqlite_fts5` tag is needed for the database's title search (see
 (the server and every enabled source's fetcher) as a `job-search` system
 user, from `/usr/local/bin/job-search` with `/etc/job-search/config.yaml`.
 Its comments give the install steps. It keeps its data in
-`/var/lib/job-search` and its log in `/var/log/job-search`, the only places
-it can write, so set `database.file` and `logging.file` there.
+`/var/lib/job-search` and any log file in `/var/log/job-search`, the only
+places it can write, so set `database.file` there. Logs go to the journal
+(`journalctl -u job-search`) unless `logging.file` is set to a file in
+`/var/log/job-search`, which then needs rotating (below).
 
 - The database isn't migrated on start: run `job-search migrate up`
   (`--create` the first time) as the `job-search` user before the first