Add digest mode setting

[n3w4z4]

Is your feature request related to a problem? Please describe.
There are 2 types of alert aggrupation "batch mode" and "per result" mode.
The templates do not contemplate the "per result" mode.

Describe the solution you'd like
Please, add a new properly in the yml files so the diggestion mode of the alert can be selected.
Setting is called alert.digest_mode in savedsearches.conf

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[AndreiBanaru]

I believe I already opened this as #445 and a potential PR #446