feat(router-store): add selectQueryParam and selectRouteParam

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

NgRx 8 gave us frequently used router selectors, selectQueryParams and selectRouteParams were two of these. Often a specific param is needed instead of all the params provided, and this selector has to be created manually:

export const selectRouteId = createSelector(
  selectRouteParams,
  params => params && params['id']
);

Closes #

What is the new behavior?

This PR adds selectQueryParam and selectRouteParam, to create a selector to retrieve a specific param from the url.

Usage:

const {
  selectQueryParam,     // factory function to select a query param
  selectRouteParam,     // factory function to select a route param
} = getSelectors(selectRouter);


export const selectRouteId = selectRouteParam('id');
export const selectStatus = selectQueryParam('status');

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 81d1bc3 at https://previews.ngrx.io/pr2014-81d1bc3/

[Odonno]

That is amazing!