Trigger on every character input instead of @ sign

Hi,

I'm using `vue-mention` to implement a textarea for inputting a mathematical expression. When the user inputs an existing variable name, the tooltip should pop up with autocomplete-suggestions just as it happens in every IDE for coding.
Currently, I only got it working with typing the `@` sign when wanting to select a variable.

But obviously it would be more convenient if the mentionable tooltip would trigger while typing simple characters (i.e. trigger on the first input character, then filter on the following ones), instead of having to use the `@` sign. I tried with using the whole alphabet as `keys` for the `Mentionable`:
```
alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".split('');
```
and
```

...
```

But that breaks the filtering, and the autocompletion doesn't work properly either.

Any suggestions on how to implement this? It would be nice to have some functionality that supports this use case.