fix: make parse options optional

[Pcrab]

The only usage of options is in line 67 yaml.load(), whose options can be an optional argument.

Make options optional can allow calling parse function with parse(str), instead of having to add an empty object parse(str, {}).

[coveralls]

Coverage: 93.514%. Remained the same when pulling 7a1d383 on Pcrab:master into 511ba4f on hexojs:master.