Add automatic completion options by default, if click-completion is installed

[tiangolo]

✨ Add automatic completion options by default, if click-completion is installed

[codecov-io]

Codecov Report

Merging #7 into master will increase coverage by 20.71%.
The diff coverage is 68.88%.

@@             Coverage Diff             @@
##           master       #7       +/-   ##
===========================================
+ Coverage   41.17%   61.89%   +20.71%     
===========================================
  Files           7        8        +1     
  Lines         391      433       +42     
===========================================
+ Hits          161      268      +107     
+ Misses        230      165       -65

Impacted Files	Coverage Δ
typer/completion.py	68.42% <68.42%> (ø)
typer/main.py	44.2% <69.23%> (+17.32%)	⬆️
typer/params.py	85.71% <0%> (+14.28%)	⬆️
typer/models.py	97.08% <0%> (+37.86%)	⬆️

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