Implemented custom indentation levels.

This is an option, IDocumentIndentEngine.EnableCustomIndentLevels has to
be set to true in order to use this feature.
Custom indents work only on statements and bodies. Indentation level of
the block body should be equal to the indentation of the block's keyword
increased by its appropriate continuation.
Merge pull request #328 from mono-soc-2013/MateY-IndentEngine

Implemented custom indentation levels.