Slim Lexer: Fix multiline ruby code

[reneklacan]

This change fixes lexing of multiline string using \ at the end of the line.

Example:

= render( \
    partial: 'foo',
    locals: { \
      foo: :bar,
    },
  )

[pyrmont]

@reneklacan The visual samples are located in spec/visual/samples. The Slim one is at spec/visual/samples/slim. If you could put an example in there as well, that'd be great.

[reneklacan]

@pyrmont Thanks. Done!

[pyrmont]

@reneklacan Thanks!