Add support for "continuation parsing"

[alexander-myltsev]

Solve https://github.com/sirthias/parboiled2/issues/26

Only subset of rules support "continuation parsing". If approach is fine then it will be extended to rest of rules.

[jonoabroad]

Is this likely to be merged into master?

[alexander-myltsev]

master is too far. Would not happen anytime soon.

[sirthias]

Closing for unlikeliness of ever reaching maturity.