Optionally allow shorthand indexes in variable paths

[jg-rp]

This PR adds a shorthand_indexes class variable to liquid.Environment. When shorthand_indexes is set to True (default is False), array indexes in variable paths need not be surrounded by square brackets.

Any custom tags that create instances of liquid.expressions.TokenStream should pass shorthand_indexes=self.env.shorthand_indexes when constructing the stream.

In a future release we'll refactor expression parsing to make sure the current environment is available to all expression parsing functions/methods so we don't need to pass a shorthand_indexes argument in a million places.

See #39.

from liquid import Environment

class MyEnv(Environment):
    shorthand_indexes = True

env = MyEnv()

data = {"foo": ["World", "Liquid"]}
template = env.from_string("Hello, {{ foo.0 }}!")
print(template) # Hello, World!

TODO:

• Docs
• Common expression parser
• Macro and call arguments