Added flag to avoid printing header

[CasperGN]

When incoorporating Webanalyzer into scripts to enumerate several items of a target the non-optional header printing was inconvinient. Hence I've created the -silent bool flag to allow for disabling the header printing.

Compile + test with and without -silent option:

~/dev/webanalyze (optional_header) $ go build -o webanalyze cmd/webanalyze/main.go 
root@kali:~/dev/webanalyze (optional_header) $ ./webanalyze 
Usage of ./webanalyze:
  -apps string
        app definition file. (default "apps.json")
  -crawl int
        links to follow from the root page (default 0)
  -host string
        single host to test
  -hosts string
        filename with hosts, one host per line.
  -output string
        output format (stdout|csv|json) (default "stdout")
  -search
        searches all urls with same base domain (i.e. example.com and sub.example.com) (default true)
  -silent
        avoid printing header (default false)
  -update
        update apps file
  -worker int
        number of worker (default 4)
~/dev/webanalyze (optional_header) $ ./webanalyze -update
2020/04/10 03:46:52 app definition file updated from  https://raw.githubusercontent.com/AliasIO/Wappalyzer/master/src/apps.json
~/dev/webanalyze (optional_header) $ ./webanalyze -host robinverton.de -crawl 1
 :: webanalyze        : v1.0
 :: workers           : 4
 :: apps              : apps.json
 :: crawl count       : 1
 :: search subdomains : true

http://robinverton.de (0.4s):
    Netlify,  (PaaS, CDN)
~/dev/webanalyze (optional_header) $ ./webanalyze -host robinverton.de -crawl 1 -silent
http://robinverton.de (0.4s):
    Netlify,  (PaaS, CDN)

[rverton]

Thanks for your contribution!

[CasperGN]

My pleasure - thanks for PR acceptance and a great project!