Support AWS filter conditions from CLI flags

Fix concurrency issue when writing to file

When writing entries from multiple providers, it will now collect them
and at the end write them to the file. Before, the last provider to
fetch the entries would overwerite the entries from the previous
providers.

Execute AWS and GCP providers concurrently

Implement GCP cloud provider

Refactor CLI interface and locked dependencies using `dep`.

Fix permissions being changed for hostsfile

Print informative message when updating a hostsfile

Add documentation

First commit