Support custom parameter merging behavior

[pangolingo]

There are a few issues in this repo reporting problems or unexpected behavior related to how parameters are merged into factories. Under the hood Fishery uses a deep-merge behavior. It uses mergeWith from Lodash and adding some customizations to the merge behavior (explicit undefined will override params instead of being ignored). It also does some fancy Typescript typing to explain how the parameters are deep-merged.

Issues related to parameter merging behavior:

• Allow specifying the type of params #108 - DeepPartial TS types causing trouble with Dates and other classes
• Invalid object produced for optional properties that have multiple nested required properties #122 - the TS types resulting from the merge are not accurate
• Factory produces data incompatible to its type constraint #106 - deep merging with explicit undefined values gives unexpected TS types
• Remove existing value with undefined #44 - desire for more control over the merge behavior (resolved by adding explicit undefined handling)
• Object properties are merged instead overriden #113 - unexpected deep merge behavior
• Symbol properties inside objects passed to the build function as associations are not getting merged into result #105 - unexpected Lodash mergeWith behavior

I propose allowing Fishery users to override the default merge behavior with their own merge strategies. This should give more control over merge behavior. Perhaps you could choose from deep merge, shallow merge, or a custom merge. This might also give more control over Typescript types, by specifying the input and output types of the merge function.