config.refactor #1

Rename struct types and members for the alertmanager and simplepush
endpoints to receiver and sender endpoints, respectively. Currently the
only viable receiver is alertmanager, and the only viable sender is
simplepush, but this change is the first in paving the way towards
making the endpoints more generic and allowing for other alerting
combinations of receiver and sender.

This is a major refactor of the code that queries and sets configuration
parameters. Functions were simplified and separated into files that
match their purpose. Rather than manually code various parameters and
their handling (setting of defaults, CLI flag options, etc.), custom
tags were implemented for the Config struct members. The options and
their settings are now all parsed from a single, unified interface (the
Config struct itself and its tags) so there is easier maintainability
going forward.

Update the sample config file to reflect the changes in generalized
endpoints, where each endpoint is either a receiver or sender, with
default types for each (alertmanager for receiver and simplepush for
sender).

Align the README.md config descriptions with the new receiver/sender
changes to the config handling.

Correct the environment variables and flags from alertmanager or simplepush
to receiver or sender, respectively.

Rename all config "pass" variable/value instances to "password".