Minor change to add support for usage with `saslauthd.conf` which differs with `:` for it's key/value delimiter.

Also adopts the`_escape_for_sed()` method, instead of the inline sed pattern (_which for some reason escaped `=` but not `(` + `)`, thus buggy if ever matching on input with those tokens_).

`_escape_for_sed()` likewise wasn't escaping for `(` + `)` or even `|`, which are required for proper escape support if using `sed -E` / `sed -r` with this method. Additionally added escaping support for `&` replacement segment token, which seems valid. Increased verbosity to better grok pattern matching expression, clarified escaping concern with `sed` delimiter since the project is not consistent there.

No need to check for a non-empty value to prepend a space (_since an empty value is used via sed anyway?_).

Can also be a bit DRY with the sed pattern, matching the key + delimiter (_and all white-space before/after the delimiter until the value_), then capture that for the replacement left-side value while only actually swapping the value for the ENV input value.

Should be an improvement, unless there is a scenario that would differ between `` and ` ` as valid value assignments?