File target: auto add .zip to compressed archive when archiveName isn't specified.

[bhaeussermann]

Fixes #932

[codecov-io]

Current coverage is 70.69%

Merging #995 into master will increase coverage by +0.03% as of f7dd4ad

@@            master    #995   diff @@
======================================
  Files          262     262       
  Stmts        14793   14793       
  Branches      1613    1614     +1
  Methods          0       0       
======================================
+ Hit          10453   10458     +5
+ Partial        400     399     -1
+ Missed        3940    3936     -4

Review entire Coverage Diff as of f7dd4ad

---

Uncovered Suggestions

1. +0.09% via ...oredConsoleTarget.cs#232...244
2. +0.08% via ...c/NLog/LogFactory.cs#180...191
3. +0.07% via ...anceCounterTarget.cs#308...318
4. See 7 more...

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[304NotModified]

Thanks! I will include this on also in 4.2 (release today)

PS: please add the Fixes #932 in the body and not in the title. It doens't work in the title...