Begin adding attachments support

[flimzy]

This handles most cases, but there are a few corner cases still to test for and maybe add support for.

[codecov]

Codecov Report

Attention: 69 lines in your changes are missing coverage. Please review.

Comparison is base (3f875c1) 63.63% compared to head (246a804) 63.81%.

Files	Patch %	Lines
x/sqlite/get.go	88.43%	24 Missing and 13 partials ⚠️
x/sqlite/put.go	81.92%	15 Missing and 15 partials ⚠️
x/sqlite/json.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
+ Coverage   63.63%   63.81%   +0.17%     
==========================================
  Files         287      290       +3     
  Lines       19223    19381     +158     
==========================================
+ Hits        12233    12368     +135     
- Misses       6417     6433      +16     
- Partials      573      580       +7     

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