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')
ok sure. although specific examples or references would be useful if there is a real problem here, so that myself and others could learn the lesson (I've never heard that Hash#fetch has threading issues, and this global state isn't thread safe anyway).