added _TabularReader.open() context manager
added IStreamFilter class
IStreamZlib fix for short block sizes
IStreamZlib optimization
fixed typo in FixedWidthWriter
added _TabularWriter.open() context manager
modified signature for _TabularReader.open()
modified signature for _TabularWriter.open()
refactored _TabularReader.open()
replaced IFileSequence with ReaderSequence
code and comment tweaks for filter.py module
WhitelistFilter and BlacklistFilter replaced with FieldFilter
added TextFilter class
updated tutorial.md
FieldFilter handles missing field
modified tutorial.md [ci skip]
refactored IStreamZlib unit tests
tweaked exception handling for Reader/Writer open() method
IStreamBuffer renamed to BufferedIStream:
IStreamFilter renamed to FilteredIStream