Adding support for Meson.

[Type1J]

Although the use of this library is very convenient by just copying over the header to another project, this makes it easy for those who want to checkout a particular commit or the HEAD at the build time of their project when they use the Meson Build System.

It adds only 1 file (meson.build), and some usage info in the README.md file.

Meson is a C++ build system, like CMake, but simpler. It aims to cut down on the time spent working on build files, and the time spent compiling.

[coveralls]

Coverage remained the same at 99.722% when pulling 31fe2f5 on Type1J:develop into 962da00 on nlohmann:develop.

[Type1J]

I'm not sure what happened with CI. I didn't change the .hpp files, just the README.md file, and I added meson.build, which shouldn't impact the build that's currently in place.

[nlohmann]

No worries - Travis is sometimes a bit buggy. I restarted the failed job. It should be fine.

[Type1J]

I think Travis is having a bad day.

[nlohmann]

I think this text is too long. It reads more than an advertisement for Meson than as a brief description how to use the library if you are using Meson. Compare this to the description of Homebrew above. As I never used Meson before, it's hard to judge what the "minimal" description should be though.

[Type1J]

The commits that I made per your review only touched the README.md file, but Travis CI is still not working correctly, it seems.

[nlohmann]

As I said, Travis is sometimes a bit buggy.

[coveralls]

Coverage remained the same at 99.722% when pulling 87744fd on Type1J:develop into 962da00 on nlohmann:develop.

[coveralls]

Coverage remained the same at 99.722% when pulling 87744fd on Type1J:develop into 962da00 on nlohmann:develop.

[nlohmann]

Thanks!