feat: add status command

[lukeed]

The status command (available via both CLI and API) checks for outstanding migration files.
In other words, how many up migrations have not yet been applied.

The following screenshots are taken with an empty database as my starting point:

Note: The node bin is identical to ley command... local dev.

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I also cleaned up the README a bit. Repeating the same opts.cwd, ..., opts.cwd was bugging me.

[codecov-commenter]

Codecov Report

Merging #10 into master will decrease coverage by 2.13%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   48.07%   45.94%   -2.14%     
==========================================
  Files           2        2              
  Lines         104      111       +7     
==========================================
+ Hits           50       51       +1     
- Misses         54       60       +6     

Impacted Files	Coverage Δ
index.js	11.76% <14.28%> (+0.28%)	⬆️

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[lukeed]

/cc @TehShrike