Added RTU modbus configs for RTS and delay

[lmartorella]

Added extra arguments for the RTU settings of the modbus line:

• serial mode
• RTS mode
• RTS delay

[BlackZork]

Merged, added new config vars to README.md.

I changed rtu_rts_delay to rtu_rts_delay_us. There is a parser for string timespans for refresh value, but I think it is overkill to use it here.

Thanks!

[lmartorella]

Great thanks!