Add Alertmanager to the OSS-Fuzz project

Proposal

prometheus/prometheus is already part of OSS-Fuzz, see here. We would also benefit from fuzzing in some parts of Alertmanager. This is easy to appreciate if one adds a simple fuzzer, which tests the config.Load function, where a fuzzer can easily detect bugs. An example

global:
route:
receivers:
- name: "example"
  webhook_configs:
  -

Results in

./amtool check-config --output=json config/testdata/conf.crash.yaml
Checking 'config/testdata/conf.crash.yaml'panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xae3398]

goroutine 1 [running]:
gopkg.in/yaml%2ev2.handleErr(0xc000311a28)