Fix custom styles w/ numFmtId == 164

According to the documentation, custom style types are delineated
starting *at* numFmtId 164, not greater than 164.

The sample xlsx included in the tests inspired the previous greater than
check, which recorded the first numFmtId at 165. However, some
in-the-wild spreadsheets do start at 164, which exposed the error.
Parse empty sheets

Previously, reading the dimensions of an empty sheet threw an error,
preventing parsing. Fixed.
Support number types styled as dates

The documentation on exactly what takes priority in terms of styles and
types together couldn't be found, but in the wild 'number' types can be
styled as dates, and excel will indeed read them as dates and not
numbers. In particular, SimpleXlsxWriter uses this format, which is now
supported.
Formula support

Previously, formulas would blow things up. Now, we correctly parse
formula values.