Accept runtime config in either JSON or TOML

[lann]

Many environments have better JSON tooling than TOML. To ease integration, accept either format for runtime config.

This chooses the format as follows:

• If the extension is .toml, parse as TOML
• If the extension is .json, parse as JSON
• If the first non-whitespace character is {, parse as JSON (never valid TOML)
• Otherwise, parse as TOML (compatible with current behavior)

[endocrimes]

GitHub mobile is too easy to fat-finger, sorry!

(Yay I am excited to purge a bunch of SpinKube code at some point)