Feat #30 add params onBeforeReplace and onBeforeSave

[abernh]

Problem

is already discussed in #30 - if the store structure changes, an old structure might be pulled into the store, causing new functionality to break.

Solution: new parameters onBeforeReplace and onBeforeSave

Allowing to "hook" into the process via parameters onBeforeReplace and onBeforeSave, functions which both get the current state as parameter, allows the developer to make adjustments before the state is altered.

Functionality

onBeforeReplace(state): state|falsy

Is triggered with the restored-from-localstorage state.
If a falsy value is returned, the store.replaceState will not be executed.

This allows e.g. to validate a state before it is pushed into the store.
Another usecase would be removing of some parts of the store state.

onBeforeSave(state): state|falsy

Is triggered after filtering and before state is saved to local storage.
If a falsyvalue is returned, the localStorage.setItem will not be executed.

This allows e.g. to add additional information to the state, e.g. a time flag state.__time = Date.now(), which in return can be validated (and filtered out again) in onBeforeReplace

[codecov]

Codecov Report

Merging #65 (a9b8c08) into master (1fb073c) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   98.24%   98.48%   +0.23%     
==========================================
  Files           2        2              
  Lines          57       66       +9     
  Branches       10       14       +4     
==========================================
+ Hits           56       65       +9     
  Partials        1        1              

Impacted Files	Coverage Δ
src/index.ts	100.00% <100.00%> (ø)

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[gabrielmbmb]

Hey @abernh , thank you so much for your PRs. Nice work!