Topic link pluralization

Unlike local links, it is not possible to automatically determine the exact pluralization of a topic link because:

it would require communicating with the OurBigBook Web API, which we could in principle do, but we would rather not have static builds depend on Web instances
topics can be written by multiple authors, and there could be both plural and singular versions of each topic ID, which makes it hard to determine which one is "correct"

Therefore, it is up to authors to specifically specify the desired pluralization of their topic links:

by default, topic IDs are automatically singularized, e.g.:
```
<#Many Dogs>
```
renders something like:
```
\a[https://ourbigbook.com/go/topic/many-dog][Many Dogs]
```
to prevent this automatic singularization, use \x p argument with {p=1}, e.g.:
```
<#Many Dogs>{p=1}
```
renders something like:
```
\a[https://ourbigbook.com/go/topic/many-dogs][Many Dogs]
```

 Ancestors