enable grid to build a query string from attributes

[guruofgentoo]

Useful in non-request contexts to generate a URL to preset filters/sort/paging/search.

fixes #162

[codecov[bot]]

Codecov Report

Merging #164 (ff97cf8) into master (0b86982) will decrease coverage by 0.31%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   90.04%   89.72%   -0.32%     
==========================================
  Files          26       26              
  Lines        6246     6251       +5     
  Branches      746      519     -227     
==========================================
- Hits         5624     5609      -15     
- Misses        536      557      +21     
+ Partials       86       85       -1     

Impacted Files	Coverage Δ
webgrid/tests/test_unit.py	97.36% <100.00%> (+0.01%)	:arrow_up:
webgrid_ta/extensions.py	42.30% <0.00%> (-42.31%)	:arrow_down:
webgrid_ta/app.py	78.94% <0.00%> (-13.16%)	:arrow_down:
webgrid/flask.py	97.16% <0.00%> (-1.89%)	:arrow_down:
webgrid/renderers.py	91.33% <0.00%> (-0.51%)	:arrow_down:
webgrid/tests/test_columns.py	98.20% <0.00%> (+0.71%)	:arrow_up:

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