added builders for week indicator and for day header

[TatsuUkraine]

Closes: #27

Added 2 builders for header:
weekIndicatorBuilder and dayHeaderBuilder

Both builder signed with same typedef, which is

typedef HeaderBuilder = Widget Function(
  BuildContext context,
  LocalDate date
);

Both builders are optional. If builder is not provided or it returns null - default (current one) header widgets will be used.

Checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[JonasWanke]

Thanks for creating this PR! I like the feature and its implementation; the comments are just about naming and code formatting

[TatsuUkraine]

Will do, thanks. I just can't get why code check fails(

[JonasWanke]

Both failing checks try to create a comment on this PR for which they need an access token. The two required tokens are stored as secrets in this repo but can't be accessed when the GitHub actions are executed on a fork. You ignore their failure (flutter analyze doesn't report errors locally).

(I'll have to find a workaround for this problem as soon as I have a bit more spare time.)

[TatsuUkraine]

all items are fixed, except this one

[JonasWanke]

This is now published as part of v0.2.6. Thanks for contributing!