Add template functions

[bojand]

Adds template functions:

newUUID - Generates a new UUID for each invocation.

randomString - Generates a new random string for each incovation. Accepts a length parameter. If the argument is <= 0 then a ranom string is generated with a random length between length of 2 and 16.

Example usage with data:

-d '{"order_id":"{{newUUID}}", "item_id":"{{newUUID}}", "sku":"{{randomString 8 }}", "product_name":"{{ranomdString 0}}"}'

Would result in data with JSON representation:

{
  "order_id": "3974e7b3-5946-4df5-bed3-8c3dc9a0be19",
  "item_id": "cd9c2604-cd9b-43a8-9cbb-d1ad26ca93a4",
  "sku": "HlFTAxcm",
  "product_name": "xg3NEC"
}

Also adds WithTemplateFuncs() API option.

Addresses #213.