Strikethrough not working

1. goto [Snarky](https://snarky.surge.sh/)
2. attempt to format text with strike through via `~`

## Results
it renders the `~` as regular text... _should_ be ~strike through~

Failing test...

```javascript
it('parses strike through', () => {
expect(snarkdown('I like ~HUGE~ tiny libraries')).to.equal('I like HUGE tiny libraries');
});
```

Start with the failing JavaScript test shown in the issue and trace the snarkdown parser entry point. Confirm the current output for `~HUGE~`; done means the parser produces the expected `HUGE` markup and the test passes.

索引モデルが issue の本文から書いたものです。