chel.toml validation

[taoeffect]

Problem

The chel.toml file currently isn't validated very well in some parts, and for other parts (like issue #104), the error messages for existing validation aren't very helpful.

Solution

• Provide validation for the chel.toml file in the sense that if parameters are specified, they contain correct values
• If keys are valid but the values are invalid (e.g. localhost: 6379 for url instead of redis://localhost: 6379), provide helpful error messages like "url" must begin with redis://, rediss://, or unix://)
• If keys are invalid (don't exist), a warning or an error should be printed. It might be worth printing an error if mandatory keys that are expected aren't defined, and a warning for other keys that are unknown (as this could indicate a typo)
• The uncaught (in promise) issue as noted in Trouble running chel serve #104 should be fixed.

[taoeffect]

Partially addressed, see: #111 (review)