Add ?_cache_size=[..] to connection parameters

[arp242]

Add a shortcut for PRAGMA cache_size; this is a pretty useful setting:
the default of -2000 (2M) is not especially high, and a lot of people
will probably want to increase this.

For example, while running a bunch of fairy expensive queries in
parallel:

With SetMaxOpenConns(1):
 -2000:  5762ms
-20000:  4714ms

With SetMaxOpenConns(20):
 -2000:  3067ms
-20000:  2532ms

Which isn't a bad performance boost for changing a single number.

[codecov-io]

Codecov Report

Merging #894 (c240601) into master (223d277) will decrease coverage by 0.25%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##           master     #894      +/-   ##
==========================================
- Coverage   46.92%   46.67%   -0.26%     
==========================================
  Files          11       11              
  Lines        1447     1457      +10     
==========================================
+ Hits          679      680       +1     
- Misses        635      642       +7     
- Partials      133      135       +2     

Impacted Files	Coverage Δ
sqlite3.go	52.90% <10.00%> (-0.49%)	⬇️

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[mattn]

Thank you