add support for formatting maps

[mgred]

Purpose (TL;DR) - mandatory

This PR adds support for formatting Map.

❗️ This waits for sinonjs/samsam#124 to use samsam.isMap in https://github.com/sinonjs/formatio/pull/51/files#diff-f62046d6a7ef22f4caadca5534824f25R119

How to verify - mandatory

1. Check out this branch
2. npm install
3. npm test

Checklist for author

• [x] npm run lint passes
• [x] References to standard library functions are cached.

[codecov[bot]]

Codecov Report

Merging #51 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #51   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         124    128    +4     
=====================================
+ Hits          124    128    +4

Flag	Coverage Δ
#unit	100% <100%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
lib/formatio.js	100% <100%> (ø)	:arrow_up:

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[mgred]

Thanks a lot @mroderick 👍