document: extract text from pdf page
Update README
Rename pageText to pageExtractText
viewer: handle WinAnsiEncoding and MacRomanEncoding
minor style fix
content processor: store glyphs by slices

Slice is a list of glyphs drawn in one shot. One may need to add
spaces or newlines between slices.
pageExtractText: handle WinAnsi and MacRoman encodings
viewer: assert not implemented text operators

Newer saw them in wild, so I want to know when
I find pdf file with them
content processor: handle character spacing
content processor: handle word spacing
Remove outdated comment
Use Int for glyph codes instead of ByteString