Add random hash generator to templating engine

[moenka]

Please add a random hash generator for the templating engine since sometimes it's necessary to create hashes for configurations (e.g. Wordpress secret keys). The function name is randomHash.

The generator should take the arguments:

• mandatory: length of hash
• optional: regexp of symbols to use for the hash - if this is not set, use all symbols available in utf-8

As a bonus the function must also work with the default function.

Example:
{{ randomHash "32" "[a-zA-Z0-9\[\]\{\}\.]" }}
{{ default .Env.AUTH_KEY randomHash "40" }}