feat: add a props to disable the divider

[SuicaLondon]

Hi

I have added a new props showDivider to disable the the default divider which hover the picker. There were two issues #102 and #106 relative to this problems at the moment, so I think it may be better to add a switch hide the lines.

Also, I think it would be better to provide some customisation for the divider later.

import { useState } from 'react'
import Picker from 'react-mobile-picker'

function MyPicker() {
  // ......

  return (
    <Picker 
      {/* ... */}
      showDivider={false}
    >
      {/* ... */}
    </Picker>
  )
}

[vercel]

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