Add optional custom elements for VT information.

[janowagner]

This allows osdp scanners to add arbitrary custom elements to a VT
information.
Some elements such as "name" (some more in the future) are
predefined. But some scanners might need to manage addtional
and very individual elements.

ospd-based scanners can use add_vt() to add (optionally) also custom data.
Next, they can implement the function get_custom_vt_as_xml_str() to
create a xml string represenation of whatever the custom content is.

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 0.2%.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##           master      #15     +/-   ##
=========================================
+ Coverage   55.54%   55.74%   +0.2%     
=========================================
  Files           4        4             
  Lines        1037     1044      +7     
=========================================
+ Hits          576      582      +6     
- Misses        461      462      +1

Impacted Files	Coverage Δ
ospd/ospd.py	61.79% <83.33%> (+0.29%)	⬆️

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