[WIP] engine.py: Add two default extensions

[ayan-b]

The extensions are expression-statement and loop-controls

Related to #90

Before raising the PR, here is a check list:

1. have you written unit tests for your code changes?
2. have you run "make format"?
3. are you requesting to "dev"?
4. have you updated the change log?
5. do you think that you can understand your changes after 6 month?
   5.1) can someone else understand your changes without your explanation?
6. are you pround of your code changes?
   6.1) do you have the feeling of achievement?
7. please add your name and github link to contributors.rst in alphabetical order.

[codecov-io]

Codecov Report

Merging #163 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #163      +/-   ##
==========================================
+ Coverage   97.79%   97.79%   +<.01%     
==========================================
  Files          40       40              
  Lines        2043     2044       +1     
==========================================
+ Hits         1998     1999       +1     
  Misses         45       45

Impacted Files	Coverage Δ
moban/jinja2/engine.py	100% <100%> (ø)	⬆️

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[chfw]

one step towards the goal.