Add AllowClear Button (type search) and allow empty value

[JonathanSchndr]

Hello everyone 👋,

I think this plugin is great so I removed 2 issue and added 1 new feature. For both issues I updated the docs.

1. fix: docs now load the local dist folder, for better development and downgrade docs to CDN Vue 2
2. new: AllowClear allows to change the input type to search and thus get a clear button (HTML default)
3. fix: It is possible to empty the input ('') and thus get a really empty string.

To point 3:
It was already possible to define an emptyValue: ''. Unfortunately this did not work. This is solved by the PR.
Reason: The Process method has overwritten by the update this again and again with 0. Likewise also the Focus Handler. The Lib Accounting-JS, for formatting, also made from a '' always 0. The bugfix now always checks the type before. If it should be an empty string, it returns this directly. In addition, it has also been removed that a 0 always becomes NULL automatically to prevent the user from really wanting to enter a 0 as a number.

Looking forward to your feedback. 🙏

This PR should resolve the following issues: #117 #57

[JonathanSchndr]

This PR is very important for further work. Alternatively, I am happy to set up a vue-numeric2, if there is no more time to continue this project?

[kevinongko]

Merged, thanks for the PR