Organize --help

[phillip-stephens]

It's always bothered me that the --help just didn't group annotation module flags together in separate sections.

This PR addresses that by dynamically constructing a --help(called Usage by the FlagSet library) string and grouping related flags together.

Old --help

Usage of flags:
  -censys
        censys internet intelligence
  -censys-pat string
        censys API personal access token (PAT)
  -censys-threads int
        how many enrichment threads to use. Note that free plan only allows 1 concurrent API request at a time (default 1)
  -geoasn
        annotate with Maxmind GeoLite/GeoIP ASN data
  -geoasn-database string
        path to Maxmind ASN database
  -geoasn-mode string
        how to open database: 'mmap' or 'memory' (default "mmap")
  -geoasn-threads int
        how many geoASN processing threads to use (default 5)
  -geoip2
        annotate with Maxmind GeoIP2/GeoLite data
  -geoip2-database string
        path to MaxMind GeoIP2/GeoLite database
  -geoip2-fields string
        city, continent, country, location, postal, registered_country, subdivisions, traits (default "*")
  -geoip2-language string
        what language geoip2 database is in (default "en")
  -geoip2-mode string
        how to open database: 'mmap' or 'memory' (default "mmap")
  -geoip2-threads int
        how many geoIP processing threads to use (default 5)
  ...

New --help

Usage: ./zannotate [global options] <-module [module-options]>...
At least one annotation module must be specified (e.g. -geoip2, -rdns, -routing).


Global Options:
  -input-decode-threads int
        number of golang processes to decode input data (e.g., json) (default 3)
  -input-file string
        ip addresses to read, use '-' for std in (default "-")
  -input-file-type string
        ips, csv, json (default "ips")
  -input-ip-field string
        key in JSON or column in CSV that contains IP address (default "ip")
  -log-file string
        where should JSON logs be saved
  -output-annotation-field string
        key that metadata is injected at, used for both CSV and JSON file inputs to preserve data in the input file (default "zannotate")
  -output-encode-threads int
        number of golang processes to encode output data (e.g., json) (default 3)
  -output-file string
        where should JSON output be saved, use '-' for stdout (default "-")
  -verbosity int
        log verbosity: 1 (lowest)--5 (highest) (default 3)

Censys Options:
  -censys
        censys internet intelligence
  -censys-pat string
        censys API personal access token (PAT)
  -censys-threads int
        how many enrichment threads to use. Note that free plan only allows 1 concurrent API request at a time (default 1)

GeoIP2 Options:
  -geoip2
        annotate with Maxmind GeoIP2/GeoLite data
  -geoip2-database string
        path to MaxMind GeoIP2/GeoLite database
  -geoip2-fields string
        city, continent, country, location, postal, registered_country, subdivisions, traits (default "*")
  -geoip2-language string
        what language geoip2 database is in (default "en")
  -geoip2-mode string
        how to open database: 'mmap' or 'memory' (default "mmap")
  -geoip2-threads int
        how many geoIP processing threads to use (default 5)