Support for `nosync` option, and increase map size (#89)

[kevinkle]

• Set readyonly and noreadyahead = True by default, and set readonly=True on cli queries

• Check that the db file exists before setting readonly

• Check that the db file exists before setting readonly

• Bump mapsize

[codecov[bot]]

Codecov Report

Merging #90 into master will decrease coverage by 0.04%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage    76.7%   76.66%   -0.05%     
==========================================
  Files          16       16              
  Lines         644      647       +3     
==========================================
+ Hits          494      496       +2     
- Misses        150      151       +1

Impacted Files	Coverage Δ
prairiedog/lemon_graph.py	87.56% <100%> (ø)	:arrow_up:
prairiedog/cli.py	86.66% <80%> (-5%)	:arrow_down:

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