Add support for codenames and very basic support for chart sheet

[ridiimer]

• Parse codename (https://docs.microsoft.com/en-us/openspecs/office_file_formats/ms-xls/848c71f8-6ad2-48e8-8572-4daef6e58f88) property for worksheets.
• Also add very basic support for reading chartsheets in on_demand mode

[coveralls]

Coverage decreased (-0.3%) to 61.936% when pulling fe92438 on ridiimer:master into a7a2247 on python-excel:master.

[cjw296]

You need to both get the existing tests to pass and add news ones for your proposed changes.

[cjw296]

Causes tests to fail and has no tests of its own.