Add option in meson to install helper application

Added two new options in meson that allow choosing whether the helper applications (gplugin-gtk-viewer and gplugin-query) are going to be installed or not

Testing Done:
Compiled a few times switching the two new options between true and false and verifying that the behavior was correct.

Bugs closed: GPLUGIN-129

Reviewed at
There are a few notes about building on OSX.
First off, building has *ONLY* been tested against homebrew. If you want to
support fink or macports, please let me know and I will merge your pull
Currently there is no known way to build the Perl loader on MacOS. So you
**MUST** run meson with `-Dperl5=false`.
## Lua
For the Lua loader to work, you need to install `lgi` from luarocks either
systemwide or to a virtual environment created by LuaDist, vert, or something
along those lines, then you just need to run `luarocks install lgi`.
## Python3
If you're using homebrew, you need to install `pygobject3`.