Improve printing functions of `TemplateMonad`

Currently `tmPrint` always outputs on the "notices" channel. It would be nice if the user could choose the channel. Something like :
```coq
Axiom tmMsg : LogLevel.t -> forall {A}, A -> TemplateMonad unit.

Module LogLevel.
Inductive t :=
  | Debug
  | Info 
  | Notice 
  | Warning 
  | Error.
End LogLevel.
```
And then we could define `tmPrint` as : 
```coq
Definition tmPrint := tmMsg Notice.
```