Force task type declaration

[cliffano]

Bob currently supports default task type, e.g. default test is mocha.
Default task type won't stay forever as libraries come and go, something becomes popular, then someday might be unmaintained.
When Bob changes the default task type in a new version, that will break module that rely on the previous default task type. This is not only annoying, but also discourage Bob upgrade.

It's better to enforce modules to declare task types in .bob.json file, and Bob should fail fast if a task is called but the type is not declared.

[cliffano]

Cancelled.

It's more appealing to have zero-configuration (all defaults) and optional task override.
Bob v0.6.0 implement lazy install for non-default task types.

The problem with default task types will always be there. It's recommended for userland to specify task type.
v0.6.0 will provide new commands to set task type easily, e.g. 'bob set lint.type nodelint' , rather than having to modify .bob.json file manually.