Linux/Mac players need to manually open a terminal and run the installer through Mono, which is confusing for players who aren't familiar with terminals. Add install.sh (Linux) and install.command (Mac) files which validate that Mono is installed and run the installer through Mono.

Done in develop for the upcoming SMAPI 2.5. This adds install on Linux.sh and install on Mac.command files, so players on Linux/Mac can just double-click to install SMAPI.