Allow extending template behavior in plugins.

[lexbritvin]

We need a way to extend template variables, functions, and overall behavior.
I want to add custom variables like compose_dir in the template.
I want to use custom functions like isNil in the template.
I want a way to replace special characters in plugins as it's done for env and template variables now.

[davidferlay]

#84

[lexbritvin]

Will be implemented in #127