README improvements [ci skip]
Initial implementation of style/theme parsing.

Huge thanks to @SheetJS for guidance on how to do all of this!

Experimental, guarded by the cellStyles option (default: false)
Merge pull request #66 from eladxxx/master

Style/theme implementation
version bump 0.7.4: I/O improvements

- basic style + theme parsing, option .cellStyles (h/t @eladxxx)
- more XLSB writing stubs
- correct resolution of .xml/.bin files
- sheet_to_json improvements from js-xls
o opts.header = 1 for array of arrays
o opts.header = 'A' for spreadsheet column labels
o custom opts.header array for custom labels
o opts.range = n starts from row n
o opts.range = range restricts writer to work within the specified range
- Makefile adapted to work with cygwin on windows
test.js fixes for windows
enforcing 31 character sheet name restriction
version bump 0.7.5: more performance

- eliminated functional constructs in hot functions
- format try-catch block extracted into new function
- cpexcel + codepage updated to 1.2.0
- more efficient (and correct) clean implementation of RGB/HSL/tint algorithms
- xlsx binary --all option enables every extra formatting and saving option
- column widths parsed and saved (requires cellStyles:true)
IE Compatibility and notes
version bump 0.7.6: IE compatibility

- jscs linting to check for trailing comma issues (h/t @altkatz)
- IE: phased out lazy string indexing in favor of charCodeAt
- XLSX: replaced certain operations in hot functions with faster alternatives
- updated SSF to 0.7.1
- improved coverage in tests
version bump 0.7.6-a: npm is flaky [ci skip]

The original push showed up as failed, pushing again
Handle shared formulas

Retrieve shared formulas according to ECMA-376, Part 1, 18.3.1.40 f
(Formula)