Added striptags filter

[igor-sadchenko]

• Added implementation for striptags filter + tests (Implement 'striptags' filter #175)
• Entry added to the list of supported filters in README.

[codecov]

Codecov Report

Merging #177 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   88.76%   88.77%   +0.01%     
==========================================
  Files          69       69              
  Lines        8703     8715      +12     
==========================================
+ Hits         7725     7737      +12     
  Misses        978      978

Impacted Files	Coverage Δ
src/filters.h	94.73% <ø> (ø)	⬆️
src/filters.cpp	89.93% <100%> (+0.01%)	⬆️
src/string_converter_filter.cpp	93.92% <100%> (+0.39%)	⬆️
test/filters_test.cpp	100% <100%> (ø)	⬆️

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[flexferrum]

Can you please add sudo apt update command before the line 48 of this file:

Jinja2Cpp/.github/workflows/linux-build.yml

Line 48 in 2e7453b

sudo apt-get install -y cmake build-essential ${INPUT_COMPILER}

This addition should fix builds.

[flexferrum]

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