add licenseVisitor and noassertion type

[dabutvin]

adds licenseVisitor option to scan

• could plug in something like spdx-correct
• I would like to be able to at least correct casing of the identifiers within an expression

adds noassertion as a parsed type

• today the parser throws when it hits an unrecognized expression
• the thinking here is that the resulting expression could still have info
• an MIT license would still satisfy an expression like MIT OR CustomLicense
• an MIT license would not satisfy an expression like MIT AND CustomLicense

[kemitchell]

See #16

[dabutvin]

👍 I'll have a look and see if I can just get that one up to date instead and then slim this one down to licenseVisitor