optionally use event.sprintf on dictionary values

[fholzer]

Introduces new configuration parameter dynamic that will enable use of event.sprintf on replacement values. dynamic defaults to false to not impact performance for current users of the plugin.

I added tests for single_value_update.rb, but not for array_of_maps_value_update.rb and array_of_values_update.rb yet.

Resolves #80. CC @guyboertje, as he's assigned to that issue.

[fholzer]

pinging @jsvd @guyboertje