Remove superfluous p-tags around shortcodes

Remove superfluous p-tags around shortcodes

This commit replaces the regexp driven `replaceShortcodeTokens` with a handwritten one.

It wasnt't possible to handle the p-tags case without breaking performance.

This fix actually improves in that area:

```
benchmark                           old ns/op     new ns/op     delta
BenchmarkParsePage                  142738        142667        -0.05%
BenchmarkReplaceShortcodeTokens     665590        575645        -13.51%
BenchmarkShortcodeLexer             176038        181074        +2.86%

benchmark                           old allocs     new allocs     delta
BenchmarkParsePage                  87             87             +0.00%
BenchmarkReplaceShortcodeTokens     9631           9424           -2.15%
BenchmarkShortcodeLexer             274            274            +0.00%

benchmark                           old bytes     new bytes     delta
BenchmarkParsePage                  141830        141830        +0.00%
BenchmarkReplaceShortcodeTokens     52275         35219         -32.63%
BenchmarkShortcodeLexer             30177         30178         +0.00%
```

Fixes #1148