Add keyword for starting the browser

[cserteGT3]

It would prefer the open() function not starting the browser, so I added a keyword to skip that part. The default behaviour does not change. The keyword is: start_browser, I have several other ideas (open_url or open_browser or open_in_browser), but could not select from them, because the function is open() and adding a kwarg like open_... seems odd.
I have concerns about type instability at the start_browser && open_url(url) line, but don't know if it's true and/or it is a problem, as it (in theory) runs once per session.
I also didn't included it into the demo or docs, because the other kwargs are also excluded.

[codecov-io]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   70.05%   70.05%           
=======================================
  Files          12       12           
  Lines         334      334           
=======================================
  Hits          234      234           
  Misses        100      100

Impacted Files	Coverage Δ
src/servers.jl	4.54% <0%> (ø)	⬆️

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[tkoolen]

@rdeits, any comments? I think this is reasonable.

[rdeits]

This looks great to me. Thank you, and sorry for the delay in responding!