FYI I was trying to use this for a project and ran into a small issue.

Specifying bus and address arguments on the CharLCD constructor does not work - the defaults of 8 and 0x20 are always used.

This does not initialize CharLCD::_i2c (the member variable), but instead creates a local named _i2c that is discarded when the constructor returns. The member variable is then default-initialized using the default MCP23017 constructor, which specifies bus 8 and address 0x20.

This resolves the issue:

 CharLCD::CharLCD(int bus, int address)
    : _i2c(bus, address)
 {
    //...

I can make a pull request if you'd like, just let me know.