Allow configuration to be set via the API

[nathanielc]

A user should be able to configure new alert handlers without having to restart the Kapacitor instance, this allows for external system to provide sensitive credentials to Kapacitor without having to directly manage the config file.

Some questions:

• If the config is changed via the API does the config on disk get updated? If so then the config needs to be writable by the user running the Kapacitor process, which is not typical. If not, does a restart then revert the config, or does some external state get merged in?
• Do we need to be able to modify all of the config or only the alert handler related items?
• When the config changes do tasks need to be restarted in order to take advantage of the new config? Or will the change happen in place.

@pauldix Here are some of my initial thoughts on this feature. Please clarify where needed.