fixed average function name
added indirect formula
Adding writer branch to the master.
The basics on the excel writer are done.
added colors dependency
playing around with async zip
fixed/ removed testing app entries
clean up code
added test cases for basic string functions
added left, right and mid functions
Merge branch 'writer' of https://github.com/thomashoffmann1979/sheeter
added entity support for saving files
fixed formula calc on open files
removed comments
fixed interactive shell
raised revision for npm package os depending tests
fixed excel 2011 formula problem