Added meson option to select if the demo application will be installed or not

Testing Done:
1. Compiled, installed with the default options and verified that the demo application and its manpage weren't installed.

2. Deleted `build` directory, reconfigured the install option added in this MR with `meson configure -Dinstall-demo=true` and then compiled and installed. This time the demo application and its manpage were installed in my system.

Bugs closed: TALKATU-84

Reviewed at https://reviews.imfreedom.org/r/156/