Improve support for installing with cget

[pfultz2]

This improves support for installing with cget:

• Adds nlohmann/json to the requirements.txt so it will automatically be installed with inja. Although, inja can be installed without it(because it can use the embedded version), this will help avoids two version of nlohmann/json being installed if two dependencies are using it.

• Set BUILD_TESTS variable based on ctest's BUILD_TESTING variable. Cget will set this variable based on whether the user is building for tests or not. So when doing cget install pantor/inja, BUILD_TESTING is set to Off and when doing cget install pantor/inja --test it will set BUILD_TESTING to On. I kept the BUILD_TESTS variable so it wont break any backwards compatibility.

• I added a note in the readme about installing inja with cget. The previous release version of inja is not installable so doing cget install pantor/inja@v2.0.1 does not work, but it does work from the latest from master. Instead of telling users to install the latest master(ie cget install pantor/inja@master), I added a placeholder <version-tag> which can be replaced with next released version when it happens.

[pantor]

Thanks so much!