Add MCP4017, MCP4018 and MCP4019
This PR adds symbols MCP4017, MCP4018 and MCP4019. The MCP4018 symbol is based on @jgladen's commit cherry-picked from #2238 (which is based on the existing symbol MCP4013).
Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/22147a.pdf
This PR intentionally violates S3.1 (Symbol slightly off-center). The existing MCP40xx symbols violate it too.