Adding custom instructions into RI5CY core

[plumerai]

We are currently evaluating the RI5CY core for our project and the possibility of adding custom instructions is important for us. Some other cores (Sifive E31, Vexriscv) have simple and documented ways to do this, but I couldn't find the information about RI5CY. HWPEs are great but we may need a true custom instruction support for our needs.