Add support setting a basePath

[Ben Sayers]

Use cases:

• Consumer has a base path in all their pact request paths, these need to be removed before attempting to look up the path in the swagger file
• Providers may set a base path in the swagger specification, this needs to be considered when doing path lookups

[Ben Sayers]

feat: add support for swagger base path

Close #24

→ <<cset 1f9828cc59ae>>