Add flag/switch/sample template filters

Small addition, but makes and/or logic in templates straight forward.

Adds filters:

* `flag` 
    * `[flag_name]|flag:request`
* `sample`
    * `[sample_name]|sample` or `[sample_name]|sample:request`
* `switch`
    * `[switch_name]|switch`, or `[switch_name]|switch:request`

Optional request params on `sample` and `switch` are kept to maintain the pattern used in `flag`, but are unused and can be omitted