add support for Prometheus AlertManager

[varuzam]

Summary

support for Prometheus AlertManager #474

awslimitchecker \
    --alert-provider=AlertManager \
    --alert-config=endpoints=http://alertmanager1:9093,http://alertmanger2:9093 \
    --alert-config=basic_auth_username=user \
    --alert-config=basic_auth_password=pass \
    --alert-config=alert_duration=1800

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[jantman]

Apologies for letting this sit for so long. As is right now, this can't be merged until it has full test coverage.