Function isAlphabetic

isAlphabetic(text?): boolean
Returns whether all the characters in the given text is alphabetic
Parameters
- Optionaltext: string
  the text to check
Returns boolean
whether all the characters in the given text is alphabetic
Example
```
isAlphabetic('abc');    // true
isAlphabetic('abc123');    // false
isAlphabetic('123456');    // false
```