Better handling of unknown platforms

Nobody has claimed this yet.

Currently if mason is run on a platform that is not one of the supported platforms (linux or osx) then the `MASON_PLATFORM` variable will not be set at https://github.com/mapbox/mason/blob/4da5ec441b9bad3aa546c414b519fee611a77e4d/mason.sh#L9-L15. This leads to and error like:

```
./mason install geometry.hpp 0.9.1
/mason/mason.sh: line 41: MASON_PLATFORM: unbound variable
```

If the `set -u` is removed from both `mason` and `mason.sh` then other errors ensue:

```
./third_party/mason/mason install geometry.hpp 0.9.1
/Users/dane/projects/node-or-tools/third_party/mason/mason.sh: line 41: [: =: unary operator expected
/Users/dane/projects/node-or-tools/third_party/mason/mason.sh: line 72: [: =: unary operator expected
/Users/dane/projects/node-or-tools/third_party/mason/mason.sh: line 102: [: =: unary operator expected
/Users/dane/projects/node-or-tools/third_party/mason/mason.sh: line 132: [: =: unary operator expected
```

I think we should likely catch the unknown `uname` case right away and throw: "Unsupported platform". However one might argue that we could support header-only installations on unknown platforms. @kkaefer is this something you think we should do (and therefore we'd need to fix the bash logic to now blow apart in comparisons)? Or do you agree that we should throw outright?

Start with platform detection in mason.sh at lines 9-15, then inspect the comparisons and failures reported at lines 41, 72, 102, and 132. Run the mason install geometry.hpp 0.9.1 example on an unsupported platform. Done means the behavior is decided and the command either reports "Unsupported platform" or handles the proposed header-only installation path without shell errors.

Written by the indexing model from the issue text.