Combine modules for I2C and SPI digipots, add MCP4xxx0 module

Combined module for I2C and SPI MCP4xxx digipots from Microchip. 

Added support for MCP42010 and the rest of the 4xxx0 line as Gordon suggested. 

Added some caveats to ULN2003 page (They have a surprisingly high voltage drop, and don't handle current nearly as well as you'd think from the headline 500ma spec), and listed some similar parts.