Rendes a style without destroy previous ostream settings

[randydu]

The ostream previous settings should be rolled back after current style rendering.
Currently we use rang::style::reset after rendering which works fine for serialized styles but not work for embedded styles:

serialize rendering:
ostream << style-1 << style-2 << ...

embedded rendering:
ostream << style-1(style-2(style-3("hello"), "world"), "!");

The ostream setting PUSH / POP should be used to make sure the setting of outer style won't be destroyed by inner style.

[randydu]

Ref: https://gist.github.com/aaronmcdaid/968629
Push and Pop the state of an iostream in C++