Validate Glean TOML Dataset

- Add glean validate command
  - --validator-path flag customizes validator program path
  - The first option sets dataset path and defaults to '.'
- Add Glean::Validator module to encapsulate validator logic
- Validate TOML dataset with tomlv program
  - If tomlv program isn't in the user's $PATH, attempt to install it
- Download platform specific tomlv program from glean/glean-validator repo
  - Glean::Installer asks user where to install tomlv program only
    when --validator-path flag isn't set
  - Glean::Installer gracefully fails in a few cases:
    - User decides not to install tomlv program
    - Platform can't be determined or matched to tomlv program
    - Network error while downloading
- Generate new cassettes for failing specs
Prefer hash rockets
Replace magic number with intention revealing constant
Better rspec output
Merge pull request #12 from simeonwillbanks/validate

Validate Glean TOML Dataset