introduce exclude flag

[dogancanbakir]

Closes #719

$ go run . -exclude 'cdn,private-ips,http://*scanme.sh,443,1.0.0.0/24,https://www.hackerone.com' -u 'http://scanme.sh,127.0.0.1,scanme.sh:443,projectdiscovery.io,example.com,https://www.hackerone.com'

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  / /_____ _/ /____ ____  ___ _
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                projectdiscovery.io

[INF] Current katana version v1.0.5 (latest)
[INF] Skipping excluded host http://scanme.sh
[INF] Skipping excluded host 127.0.0.1
[INF] Skipping excluded host scanme.sh:443
[INF] Skipping excluded host https://www.hackerone.com
[INF] Started standard crawling for => https://projectdiscovery.io
[INF] Started standard crawling for => https://example.com
https://projectdiscovery.io
https://example.com
https://projectdiscovery.io/

Note: CDN is not excluded. I would like to discuss whether we should add a check for CDN in the network policy package or not.