Add easier bindings interface

[mcrumiller]

Right now, binding variables requires a bit of investigation into the internals:

from jsonata import Jsonata as JS

data = {
    "products": [
        {"name": "Apple", "price": 1.20},
        {"name": "Banana", "price": 0.50},
        {"name": "Cherry", "price": 2.50}
    ]
}

# It would be nice to be able to just input this dictionary.
bindings = {
    "maxPrice": 1.50,
    "currencySymbol": "$"
}

# Instead we have to create an empty Frame object and bind each item.
binding_frame = JS.Frame(None)
  for k, v in input.items():
      binding_frame.bind(k, v)

expression = jsonata.Jsonata("products[price <= $maxPrice].(name & ' costs ' & $currencySymbol & $string(price))")
result = expression.evaluate(data, bindings=binding_frame)
print(result)
# ['Apple costs $1.2', 'Banana costs $0.5']

It would be nice to simply be able to pass a dictionary as the bindings parameter to evaluate().