Validate formatting attributes

[foster999]

Users can enter formatting arguments that are not implemented in xlsxreader. These raise an error once passed to a xlsxwriter Format object init e.g.: 'Format' object has no attribute 'set_potatoe'

It would be preferable to catch these when a Theme is initialised. We should get all Format methods starting with "set_" and use this list to validate. This would support all formats in the users xlsxwriter version.

Validate formats in:

• [x] Theme init by config

• [x] Theme setting single attr

• [x] GPTable.additional_formatting

[foster999]

Added method to Theme. Could move this to util type function, for use across Theme and GPTable that take formatting data

[foster999]

Additional formatting implemented in #63, but not validated

[foster999]

Validation of GPTable additional formatting and Theme format setting implemented in #64