Fix for const char* conversion warnings.

[neltnerb]

Tested and verified on Teensy 3.6. Apologies, I don't know how to properly submit a patch but the changes are very simple.

Change function definition for SerialCommands to:
SerialCommands(Stream* serial, char* buffer, int16_t buffer_len, const char* term = "\r\n", const char* delim = " ") :

to avoid warnings related to converting a const char* to char* in the latest GCC versions. The termination character and delimiter are usually fixed after configuring, not sure if you intended the ability to change these on the fly as a feature or if that's incidental.

I also changed the private variable types for term and delim to const char* as well.

I think I probably only noticed because I set it up to give me all compiler warnings, it seemed to work fine both ways.

[yawor]

@neltnerb this has been fixed around 2017, but the fix has not been published as the version was not changed. Just check the source files.