✨ Add new target and template global variables

[CLiu13]

This adds new global variables for the target and template files.

Closes #29

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?

[codecov-io]

Codecov Report

Merging #125 into dev will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #125      +/-   ##
==========================================
+ Coverage   98.57%   98.59%   +0.01%     
==========================================
  Files          38       39       +1     
  Lines        1896     1921      +25     
==========================================
+ Hits         1869     1894      +25     
  Misses         27       27

Impacted Files	Coverage Δ
moban/engine.py	100% <100%> (ø)	⬆️
tests/fixtures/globals/variables.template	100% <100%> (ø)
tests/test_engine.py	100% <100%> (ø)	⬆️

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[jayvdb]

add a test which uses these variables in a file which has been included using {% include ... %} . i.e. test nesting of templates doesnt change the global.

[jayvdb]

We will need a few other mentors to also check this, but it looks good enough for initial approval

[chfw]

neat code. no complaints.

[chfw]

oh.. I forgot one thing:

1. update change log :)

@CLiu13 , would you like to give another PR on the change log update?

[CLiu13]

Sure thing! See #128