Add case insensitive commands

[ark-]

It would be nice if the command string wasn't case sensitive.

Something along the lines of adding it as an OPTIONAL parameter to addCommand().

This could be implemented by using strupr() or strlwr() in addCommand() when adding to the _commandList . Then use the same (lower or upper) function in the readSerial() function. Specifically here: https://github.com/Erriez/ErriezSerialTerminal/blob/a9f8a8bbe159ae85ad868b84fe4207fa29629052/src/ErriezSerialTerminal.cpp#L146

The conversion would only be carried out if configured to do so.