Use JSON for internal platform config, to avoid TOML issues

[mikrostew]

Closes #219.

[mikrostew]

Now this is what happens when I run the commands from that bug:

$ notion install node 11

$ cat ~/.notion/tools/user/platform.toml
[node]
runtime = '11.4.0'
npm = '6.4.1'

$ notion install yarn 1.9.2

$ cat ~/.notion/tools/user/platform.toml
yarn = '1.9.2'

[node]
runtime = '11.4.0'
npm = '6.4.1'

[dherman]

A couple suggestions.

Another thought that occurs to me is that TOML is optimized for human-readability, which is where some of these complexities come from. Maybe for internal files like platform.toml we'd be better off with JSON. IOW, the policy would be: TOML for human-maintained files (like the hooks configuration), and JSON for files representing internal state. Just a thought; we could also decide later.

[dherman]

Looks great!