Add yaml value validators/normalisers

[jayvdb]

One use of plugins is to validate and 'fix' yaml values.

e.g. if a value for foo was a string "abc", but template now expects a list, a normaliser plugin foo_validator could return an array with one item , ['abc']

[jayvdb]

We can start this by using jinja2schema to create list of template vars and compare with yaml vars, which is #130 .

This will be an important change to moban plugin api.

that api change should also support other validation.

[ayan-b]

Assigning to myself.

[ayan-b]

Since some part of this is not a moban exclusive problem, I think it would be a good idea to create different module altogether.