Optimize replaceShortcodeTokens

Optimize replaceShortcodeTokens

We can of course skip reading the entire byte slice again and again.

This was a slip in the original implementation; functionally the same,
but is slightly faster, esp. for larger data sets with many shortcodes:

```
benchmark                             old ns/op     new ns/op     delta
BenchmarkReplaceShortcodeTokens-4     15505         14753         -4.85%

benchmark                             old allocs     new allocs     delta
BenchmarkReplaceShortcodeTokens-4     1              1              +0.00%

benchmark                             old bytes     new bytes     delta
BenchmarkReplaceShortcodeTokens-4     3072          3072          +0.00%
```