Add completion from root command

[charphi]

Fix #1051

[codecov-commenter]

Codecov Report

Merging #1056 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1056   +/-   ##
=========================================
  Coverage     94.35%   94.35%           
  Complexity      442      442           
=========================================
  Files             2        2           
  Lines          6483     6484    +1     
  Branches       1735     1736    +1     
=========================================
+ Hits           6117     6118    +1     
  Misses           94       94           
  Partials        272      272           

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/AutoComplete.java	97.04% <100.00%> (ø)	139.00 <0.00> (ø)
src/main/java/picocli/CommandLine.java	94.19% <100.00%> (+<0.01%)	303.00 <0.00> (ø)

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[remkop]

Overall looks good. I put some comments for minor improvements.

[remkop]

Can you also update the user manual to add the new ROOT-COMMAND-NAME to the variables section?

And any other documentation you can think of.

[charphi]

I've just modified index.adoc but I'm not sure what to do next since I'm not familiar with asciidoc.

[remkop]

Merged into master.
Thank you for the contribution!