Add safeHtml template function.
Add safeHtml template function.

Fixes https://github.com/prometheus/alertmanager/issues/314

sarathsp06/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,ericgerling/alertmanager,Aloomaio/alertmanager,brancz/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,Aloomaio/alertmanager,ericgerling/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,sarathsp06/alertmanager,joehandzik/alertmanager,tinytub/alertmanager,Kellel/alertmanager,brunoqc/alertmanager,tbouvet/alertmanager,tbouvet/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,brunoqc/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,brancz/alertmanager,tinytub/alertmanager,brunoqc/alertmanager,tinytub/alertmanager,tbouvet/alertmanager,brancz/alertmanager,brancz/alertmanager,sarathsp06/alertmanager,tyrken/alertmanager,Kellel/alertmanager,ericgerling/alertmanager,GoogleCloudPlatform/alertmanager,brunoqc/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager