Add :assume_from_symbol option to #parse

Ok, so we're finally upgrading to Money 6 and one issue is annoying me. I really want to use the `Monetize.assume_from_symbol` option, but not everywhere (and I don't want to audit our codebase for everywhere it could possibly affect if we change it globally).

Therefore I propose making this a method-level option. If the method level option is submitted, it will override the class level option.

Current proposal:

    Monetize.parse("£1.00", nil, assume_from_symbol: true) # => Money.new(100, 'GBP')

Alternative implementation (not in this PR, but easy to add):

    Monetize.parse_with_symbol("£1.00", nil) # => Money.new(100, 'GBP')
Good point. Updated.

Any thoughts on the `Monetize.parse_with_symbol()` syntax?