Provide a way to select specific logger channels to email

Currently, all warnings/errors from any source are emailed. This leads to a lot of unnecessary noise, such as:

Attempting to re-run cron while it is already running.

Illegal choice <em class="placeholder">field_date_value</em> in <em
class="placeholder">Sort by</em> element.

Determine a way to reliably find channels and loop over them to provide a fieldset of checkboxes on the config page to allow selection of channels for which to send emails.

OR

Provide radio buttons on the config page to use default or custom handling for channels. When custom is selected, a text area where allowed channels may be typed in, one per line.

Update the logger class to validate against this setting.