Allow filtering EIPs by AWS tag (New --tag parameter)

[hmmmsausages]

This PR, I believe, fixes: #31

Summary:

Simple feature addition, as I was facing the same issue as described in #31.
Basically I was having a similar issue, where it would be of advantage to simply assign EIPs from a pool of EIPs that have a specific AWS tag (for example Group=prod-eips) instead of specifiying the IPs directly, which would require in my case EC2 launch configuration / CloudFormation template updates every time I would add or remove new EIPs.

Usage:

The tag approach works pretty well in conjunction with auto-scaling groups.
For example execute the script during the start-up of the EC2 instance via UserData or cfn-init helper scripts, like so:

pip install aws-ec2-assign-elastic-ip
aws-ec2-assign-elastic-ip --tag Group=prod-eips >> /var/log/aws-ec2-assign-elastic-ip.log

This then allows me to manage the allocated EIPs on AWS by adding or removing a tag with the key Group and the value prod-eips. Thus when I have the need to scale up my EC2 instances, I simply make sure I have enough EIPs with the correct tag available and then perform the scaling operation without updating the EC2 launch configuration.

If I want I can narrow down the eligible EIPs by still providing --valid-ips and --invalid-ips in addition to --tag as before.

@sebdah Let me know what you think. I'm not the greatest Python dev, so feel free to refactor it, but I think this could be useful for others as well (like #31 - even though it's coming one and half years too late for him, I suppose) :D