Add argument to monitor without building/flashing

[SergioGasquez]

Add an argument that opens the serial monitor without having to go through building and flashing.

[MabezDev]

This would be useful, perhaps specifying espflash --monitor with no elf file should be a valid option. Currently it just returns the help text.

[jessebraham]

Might have to be a subcommand to make it consistent between both binaries. cargo-espflash will just build and flash the project if no subcommand is provided.

[jessebraham]

Thanks for doing this @SergioGasquez!