Add automatic tab indentation support

[Fallen-Breath]

Add automatic tab indentation support with the following steps:

1. Tweak the String.indentation to treat '\t' as a valid indentation
2. Make the indentStack store indentation strings instead of indentation levels, so the actual indentation can be preserved as-is

Mixed usage of space + tabe is untested though (do we really need to support that?)

Resolved #8