Allow reading from Excel documents

[jordangray]

Right now, the formatter can only write to Excel documents, but there's no reason it shouldn't be possible to deserialise data from an Excel document to a collection of objects.

[jordangray]

Theoretically, this should be easy:

1. Read data from stream into an Excel document with EPPlus.
2. 1. If the target type is a "simple" type, each item can be cast directly to that type.
   2. If the target type is a class, try to use headings to determine what values correspond to properties, taking into account any ExcelAttribute.Heading specified on members.
   3. If the target type is a class and there is no heading row, deserialise based on order, taking into account any ExcelAttribute.Order specified on members.
3. 1. If the target type doesn't implement IEnumerable, just deserialise the first non-header row into the target type.
   2. If the target type does implement IEnumerable, deserialise each row into an item of the target type's item type.

In practise, I'm certain that a dozen horrible and unanticipated scenarios will crop up—