fix: distinct default namespace values

In some functions, the default namespace was null, while in others was an empty
string. The default values must be consistent for the package to work when a
specific value is not provided.

feat: add MorphToMany method

replace self::class with static::class to allow overriding default class

fix bug in bulk operations when using custom Tag class

This bug is very similar to the previous one, which  was  fixed,  except
that it applies to other bulk operations, such as getting tagged models,
revoking multiple tags, etc.

fix Model name when creating tags

If users of this package use their own Tag model,  it  would  still  use
this package's Tag model on the 'model' field of Taggable class, instead
of the user's Tag model. This is now fixed.

fix bug in delAllTags

The method delAllTags was deleting the tags when it should  delete  just
the association between the model and the tags, not the tags themselves.

format README

update readme

add tests

update readme