Line continuations for indentation

Hi Steve,

This is an implementation (really for feedback at this stage) of line-continuation support, related issues are #102 and #92. 

I expect the implementation may be controversial - I've used Ripper's s-expression support rather than trying to determine expression start and end from the lexer tokens. It's also using Nokogiri (as I do elsewhere in foodcritic) to query the AST - that's unconventional and adds a significant new dependency.

Would really appreciate your feedback. 

Thanks,

Andrew.