feat: add possibility of override getTranslateFn

[rafaelss95]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/transloco/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have 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
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently we can't override the getTranslateFn as it's private.

Issue Number:
Closes #489.

What is the new behavior?

This changes getTranslateFn to be protected and so we can override it in order to make it work with the cases mentioned on #489.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[rafaelss95]

@shaharkazaz is there any release plan that includes this version? I'm willing to start using this ❤️

Oh, I just noticed that you already released a new version some minutes ago. Thank you for that :)

[angelaki]

Woooho! Timing! ;)

[angelaki]

Love the new possibilities! ❤ Here is my use-case:

protected getTranslateFn(lang: string, read: string | undefined): (key: string | ITranslateable, params?: HashMap) => any {
  const translateFn = super.getTranslateFn(lang, read);
  return (k, b) => {
    if (typeof (k) === 'string') { return translateFn(k); }
    else { return k.displayTextTranslation ? translateFn(k.displayTextTranslation) : k.displayText; }
  }
}

Pre: {{m.displayTextTranslation?t(m.displayTextTranslation):m.displayText}}
Post: {{t(m)}}