Add --duration argument to gvm-cli

[bjoernricks]

Allow to measure the duration of a xml command execution.

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #206 into master will decrease coverage by 0.59%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #206     +/-   ##
=========================================
- Coverage   33.92%   33.33%   -0.6%     
=========================================
  Files           6        6             
  Lines         395      402      +7     
=========================================
  Hits          134      134             
- Misses        261      268      +7

Impacted Files	Coverage Δ
gvmtools/cli.py	0% <0%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #206 into master will decrease coverage by 0.59%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #206     +/-   ##
=========================================
- Coverage   33.92%   33.33%   -0.6%     
=========================================
  Files           6        6             
  Lines         395      402      +7     
=========================================
  Hits          134      134             
- Misses        261      268      +7

Impacted Files	Coverage Δ
gvmtools/cli.py	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6bb9080...ca65e1c. Read the comment docs.