Add config for specifying sheets to ignore

Add config for specifying sheets to ignore in workday calculations.
Useful for easily tracking your lunches and breaks without including
them in working time.
Update documentation for day formatter
Rename config parameter

'day_ignore_sheets' seems a clumsy parameter name.  'day_exclude_sheets'
feels better.
Update README with new config parameter name
Merge pull request #2 from duckpuppy/ignore_sheets

Add config for specifying sheets to ignore