Fixes issue where deepmerge concatenates arrays inside DEFAULT_CONFIG

[jhou]

... instead of replacing them.

Description

provides deepmerge with a function to replace its typical array merging behavior with a function that overwrites the destination array with a copy of the source array instead of concatenating the two.
Documentation for that feature here: https://www.npmjs.com/package/deepmerge#arraymerge

Motivation and Context

Without this fix, the searchFields config will always have [ 'label', 'value'] plus whatever config the user passes in, making it impossible to NOT search both label and value
#495

How Has This Been Tested?

Tests included in PR. Basically, attempt to type value entries into the search input, and make sure there are no results.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project. (i hope)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[jshjohnson]

Thanks for this :)