add --allowed-hosts CLI option

[orteth01]

What kind of change does this PR introduce?
feature

Did you add or update the examples/?
no

Summary
adds the ability to set the allowedHosts option via the CLI

Does this PR introduce a breaking change?
no

Usage
I'm not sure what the best practice is for passing a list as a command line option. I opted for a comma delimited string. E.G.

webpack-dev-server --entry /entry/file --output-path /output/path --allowed-hosts host1.com,.host2.com

creates the option

allowedHosts: [
    'host1.com',
    '.host2.com'
] 

Please let me know if there is a better way to do this.

[codecov]

Codecov Report

Merging #1012 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1012   +/-   ##
=======================================
  Coverage   72.25%   72.25%           
=======================================
  Files           4        4           
  Lines         465      465           
  Branches      140      140           
=======================================
  Hits          336      336           
  Misses        129      129

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[shellscape]

Looks good, thanks!