Extensions and fixes for json_generator

[CezaryGapinski]

In this PR I try to:

• solve problem with generating output report when '--output' option is missing for json_generator
• add feature suggested in issue Aggregation of coverage measurements for code with ifdef #338 about adding wildcards to '--add-tracefile' option

I need a review for the second feature, because I am not sure whether is correctly implemented or should be done totally different.

[codecov]

Codecov Report

Merging #351 into master will increase coverage by 0.16%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   95.02%   95.18%   +0.16%     
==========================================
  Files          17       17              
  Lines        1949     1952       +3     
  Branches      338      339       +1     
==========================================
+ Hits         1852     1858       +6     
  Misses         47       47              
+ Partials       50       47       -3

Impacted Files	Coverage Δ
gcovr/configuration.py	97.27% <ø> (ø)	⬆️
gcovr/json_generator.py	97.36% <50%> (ø)	⬆️
gcovr/__main__.py	91.62% <83.33%> (+1.84%)	⬆️

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[latk]

Thank you for this great contribution! Only minor comments below.

I independently discovered and fixed the --json --output issue as part of bcb22d2 on the html-rewrite branch. However, that branch is not yet ready for merging into master so I will apply your PR first.

[CezaryGapinski]

@latk Thank you for your review. I was not aware about your fix for missing --output. I only noticed html-rewrite branch name and I have assumed that there is no changes for json output. I should check it more deeply. Sorry for that :).