Upgrade dependencies

[nicksnyder]

• Go from 1.9 to 1.12 (due to BurntSushi/toml).
• github.com/BurntSushi/toml v1.0.0
• golang.org/x/text v0.3.7

Fixes #267

[codecov]

Codecov Report

Merging #268 (8eed438) into main (240abfe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #268   +/-   ##
=======================================
  Coverage   82.40%   82.40%           
=======================================
  Files          15       15           
  Lines        1182     1182           
=======================================
  Hits          974      974           
  Misses        145      145           
  Partials       63       63           

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