Take into account several plan lines in context

[enzbang]

plan_line attribute is reused to store several plan lines separated
by ';'. Long term solution would be to rename it plan_lines and to have
a tuple of plan lines.

For #293

[codecov]

Codecov Report

Merging #294 into e3transition will increase coverage by 0.81%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##           e3transition     #294      +/-   ##
================================================
+ Coverage         89.09%   89.91%   +0.81%     
================================================
  Files                68       72       +4     
  Lines              4953     5491     +538     
================================================
+ Hits               4413     4937     +524     
- Misses              540      554      +14

Impacted Files	Coverage Δ
e3/electrolyt/plan.py	99.17% <100%> (+0.01%)	⬆️
e3/os/platform.py	95.12% <0%> (-4.88%)	⬇️
e3/os/process.py	97.33% <0%> (-1.56%)	⬇️
e3/store/cache/backends/filecache.py	100% <0%> (ø)	⬆️
e3/os/fs.py	100% <0%> (ø)	⬆️
e3/os/timezone.py	100% <0%> (ø)	⬆️
e3/os/windows/process.py	100% <0%> (ø)
e3/os/windows/fs.py	100% <0%> (ø)
e3/os/windows/__init__.py	100% <0%> (ø)
e3/os/windows/native_api.py	100% <0%> (ø)
... and 2 more

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