✨ Adding new heading configuration

[vikripermana21]

Hi Kolkov!, i wanted to request a PR, this PR provide a feature that now headings can be customized. I hope u accept this PR. Thanks

[codecov[bot]]

Codecov Report

Patch coverage: 66.66% and project coverage change: +0.12 :tada:

Comparison is base (40431f4) 20.66% compared to head (4179eef) 20.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #531 +/- ## ========================================== + Coverage 20.66% 20.79% +0.12% ========================================== Files 8 8 Lines 479 481 +2 Branches 103 104 +1 ========================================== + Hits 99 100 +1 Misses 377 377 - Partials 3 4 +1 ``` | [Impacted Files](https://app.codecov.io/gh/kolkov/angular-editor/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kolkov) | Coverage Δ | | |---|---|---| | [projects/angular-editor/src/lib/config.ts](https://app.codecov.io/gh/kolkov/angular-editor/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kolkov#diff-cHJvamVjdHMvYW5ndWxhci1lZGl0b3Ivc3JjL2xpYi9jb25maWcudHM=) | `100.00% <ø> (ø)` | | | [...angular-editor/src/lib/angular-editor.component.ts](https://app.codecov.io/gh/kolkov/angular-editor/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kolkov#diff-cHJvamVjdHMvYW5ndWxhci1lZGl0b3Ivc3JjL2xpYi9hbmd1bGFyLWVkaXRvci5jb21wb25lbnQudHM=) | `17.44% <50.00%> (+0.38%)` | :arrow_up: | | [...editor/src/lib/angular-editor-toolbar.component.ts](https://app.codecov.io/gh/kolkov/angular-editor/pull/531?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kolkov#diff-cHJvamVjdHMvYW5ndWxhci1lZGl0b3Ivc3JjL2xpYi9hbmd1bGFyLWVkaXRvci10b29sYmFyLmNvbXBvbmVudC50cw==) | `17.74% <100.00%> (ø)` | |

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