Make @df support string column names

[ron-wolf]

Fixes #562. Add a special case to add_sym! to convert an AbstractString column name to a Symbol before adding it.

The old behavior would iterate over the string and try to add each character as a column name. This produced a stack overflow, since add_sym! had no special case for Char, resulting in indefinite recursion.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 26.44%. Comparing base (bb65b7c) to head (8eaf308).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/df.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
- Coverage   26.52%   26.44%   -0.09%     
==========================================
  Files          20       20              
  Lines        1248     1248              
==========================================
- Hits          331      330       -1     
- Misses        917      918       +1     

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