search in alternate dirs for config; closes #83

[boneskull]

This will additionally search for an .rtkrc.js or rtk.config.js in $HOME, $(npm prefix)/etc, and if on POSIX OS, XDG config dir(s) and /etc, in that order.

I note that it's not particularly ergonomic to do this using cosmiconfig's API.

[codecov[bot]]

Codecov Report

Merging #84 into master will increase coverage by 0.17%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   84.16%   84.33%   +0.17%     
==========================================
  Files          51       51              
  Lines         903      913      +10     
==========================================
+ Hits          760      770      +10     
  Misses        143      143

Impacted Files	Coverage Δ
...ckages/common/test/fixture/config/custom.config.js	100% <ø> (ø)	:arrow_up:
packages/config/src/configs/recommended.js	100% <ø> (ø)	:arrow_up:
packages/inspector/src/rule-config.js	88.88% <100%> (+0.65%)	:arrow_up:
packages/core/src/observable.js	90% <100%> (+0.52%)	:arrow_up:
packages/cli/test/e2e/cli-helper.js	100% <100%> (ø)	:arrow_up:
packages/fs/src/fs-config-loader.js	91.3% <100%> (+1.3%)	:arrow_up:
packages/config/src/index.js	36.36% <100%> (ø)	:arrow_up:

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[boneskull]

Note that this changes the name of the default "recommended" config from report-toolkit:recommended to rtk:recommended. Existing configuration files will break.