Introduce 'pp' library (#430)

Introduce 'pp' library (#430)

* Introduce 'pp' library

This commit introduces 'pp' library which provides various helper
functions and classes for pretty-printing.

The current implementation provides 'fmt' function which constructs a
string using operator<<, and IndentedDocumentBuilder class which makes
it easy to build indented documents especially when indentation is
nested.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>
* Check _level before unindent

* Use static storage quantifier

* Use strict compile options

* Remove unnecessary override

* Do not construct temporary string objects