PrettyPrint implements a pretty printing algorithm. It finds line breaks and
nice indentations for grouped structure.

By default, the class assumes that primitive elements are strings and each byte
in the strings have single column in width. But it can be used for other
situations by giving suitable arguments for some methods:
- newline object and space generation block for PrettyPrint.new
- optional width argument for PrettyPrint#text
- PrettyPrint#breakable

There are several candidate uses:
- text formatting using proportional fonts
- multibyte characters which has columns different to number of bytes