[Feature Request] trim inputValue easily when cascader is `filterable`

### Existing Component
是

### Component Name
Cascader

### Description
Is there an easy way to trim the inputValue of a cascader when it is `filterable`? I know I can trim the keyword in a customized `filter-method` . But, all I want is just to trim the inputValue.

I have some ideas on making trim inputValue more easily:
1. Add a `trimInput` property in `props`
2. Use `before-filter` hook. If `before-filter` hook return a string or a Promise that resolve a string, then cascader will use this string to filter the options.

If you think this is an good idea, I can make some contributions.