[feature] spoonerism (คำผวน) function

## Detailed description
Add function to perform spoonerism on texts

## Context
Spoonerism (คำผวน) is a unique and important part of Thai language.

## Possible implementation
```
pythainlp.puan(“ประชาธิปไตย”)
> ประไชยธิปตา
```
