Add direct text migration mode

[dbendilas]

Add mode to the migrate Django command that converts code to Transifex Native syntax.

Introduced a new parameter to the ./manage.py transifex migrate command, --text or -t (this command needs to be run from inside a Django app's venv that has transifex-python installed).

HTML Django templates

./manage.py transifex migrate --text='{% blocktrans %}This string will have {{ value }} inside.{% endblocktrans %}'

./manage.py transifex migrate -t='{% t amount=article.price %}
                                 That will cost $ {amount}.
                                 {% endt %}

                                 {% t myvar=value|filter %}
                                 This will have {myvar} inside.
                                 {% endt %}'

Python/gettext code

./manage.py transifex migrate --text='from django.utils.translation import gettext as _

                                 a = _("This is translatable")'

The import statements need to be present, otherwise the function calls cannot be detected (just like when migrating Python code with the "file" mode).

[codecov]

Codecov Report

Merging #32 into devel will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            devel      #32      +/-   ##
==========================================
+ Coverage   76.72%   77.03%   +0.30%     
==========================================
  Files          33       33              
  Lines        2647     2660      +13     
==========================================
+ Hits         2031     2049      +18     
+ Misses        616      611       -5     

Impacted Files	Coverage Δ
...ransifex/native/django/management/utils/migrate.py	95.23% <100.00%> (+0.79%)	⬆️
transifex/native/tools/migrations/execution.py	75.17% <100.00%> (+0.72%)	⬆️
transifex/_version.py	44.76% <0.00%> (+1.80%)	⬆️

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