Add option to disable symbol parsing for ScalarScanner

[calebstewart]

This PR adds an option to ScalarScanner to disable the parsing of values containing a leading colon as symbols. This is unexpected behavior when compared with other YAML parsers and when compared with the YAML specification, as symbols are not a concept defined by YAML. I do not expect that we can change the default here as that would break backwards compatibility, so this PR maintains current behavior by leaving the default as parse_symbols: true, but exposes a way that users can modify the behavior in order to match what is expected based on the YAML specification.

The option is added to the load() family of methods at the top level as well as to the ToRuby visitor and the associated node to_ruby() method. In all instances, the default value of the new parse_symbols parameter is true in order to maintain backwards-compatible behavior. Passing a value of false for this parameter will result in values with leading colons being treated as regular strings.

[calebstewart]

This PR was created in response to #669. For a concrete example of this functionality, I'll use an equivalent example from that original issue:

irb(main):001:0> require 'yaml'
=> true
irb(main):002:0> a = YAML.load('servers: ::ffff:10.10.10.1')
=> {"servers"=>:":ffff:10.10.10.1"}
irb(main):003:0> b = YAML.load('servers: ::ffff:10.10.10.1', parse_symbols: false)
=> {"servers"=>"::ffff:10.10.10.1"}
irb(main):004:0>