Added support for reading usage page data from external files
Added external .conf files for each common usage page
Moved all the usage definition information from the rd.rex program into individual .conf files.

The .conf files must be called xxxx.conf where "xxxx" is the usage page
number in uppercase hexadecimal. For example, 000C.conf (not 000c.conf) for the Consumer Devices Page.

Added Usage Page 0020 (Sensor Device Page).