Add option to automatically populate field placeholders

[smithdc1]

This is my attempt to fix #786

Now... Whilst I've learnt a fair amount looking at this and it's been very enjoyable I'm not entirely sure it should be merged.

I understand that we should show restraint when adding new features, and as described in the issue there is a DRY way of doing this outside of crispy-forms. I guess at least this one shouldn't break anything.

Appreciate your thoughts, both on the topic of implementing this issue, and some feedback on my code would be nice as well (whatever the outcome).

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[codecov-io]

Codecov Report

Merging #935 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
+ Coverage    95.1%   95.12%   +0.02%     
==========================================
  Files          24       24              
  Lines        2756     2771      +15     
  Branches      252      254       +2     
==========================================
+ Hits         2621     2636      +15     
  Misses         90       90              
  Partials       45       45

Impacted Files	Coverage Δ
crispy_forms/templatetags/crispy_forms_tags.py	97.45% <ø> (ø)	⬆️
crispy_forms/tests/test_form_helper.py	97.28% <100%> (+0.03%)	⬆️
crispy_forms/templatetags/crispy_forms_field.py	86.6% <100%> (+0.75%)	⬆️
crispy_forms/helper.py	90.82% <100%> (+0.04%)	⬆️

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[carltongibson]

HI @smithdc1.

The code you have here is fine for this, but no, it's not something I think we should add.
Essentially, it's always much better to do this kind of thing explicitly. The linked example from the issue seems entirely correct to me. (Folks are welcome to add a method to their form classes, as a base class or mixin in they want it DRY™, to do this for them.)

Gonna close on that basis...