Support sql indexes.

[pwalsh]

Tests will fail without frictionlessdata/tableschema-sql-py#90

[pwalsh]

@akariv

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@400b96f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #169   +/-   ##
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  Coverage          ?   87.58%           
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  Files             ?       64           
  Lines             ?     2594           
  Branches          ?        0           
=========================================
  Hits              ?     2272           
  Misses            ?      322           
  Partials          ?        0           

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