Set arbitrary UDA via templates

It'd be great if bugwarrior would be able to set any field including UDAs using jinja templates.

For example:
```ini
# taskrc
uda.my_uda.type = string
```
```toml
# bugwarrior.toml
[gitlab]
project_template = "{{gitlabrepo}}.#{{gitlabnumber}}"  # currently works
my_uda_template  = "{{gitlabrepo}}.#{{gitlabnumber}}"  # currently does NOT works
```