Add SpreadsheetLoader support for sign-magnitude

The spreadsheet loader defaults to 2’s complement when
extracting signed integers. With this commit, the
representation can be specified between parenthesis
in the raw value column, i.e.: int(2c), int(si).
Just ‘int’ will still default to 2’s.

Bug: 2259
Merge pull request #25 from fqqb/bug2259

Add SpreadsheetLoader support for sign-magnitude