Add exact lookup type for dictionarySearch

[benlieb]

For my purposes, I needed a more exact lookup functionality.

For example:

console.log(hanzi.dictionarySearch('小孩', 'only')) gave

console.log(hanzi.dictionarySearch('小孩')) gave

But now this console.log(hanzi.dictionarySearch('小孩', 'exact')) gives

[codecov-io]

Codecov Report

Merging #60 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   87.77%   87.81%   +0.04%     
==========================================
  Files          15       15              
  Lines         548      550       +2     
  Branches       91       92       +1     
==========================================
+ Hits          481      483       +2     
  Misses         55       55              
  Partials       12       12

Impacted Files	Coverage Δ
lib/dictionary.js	88.06% <100%> (+0.07%)	⬆️

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