allow linkifying the logo

[mcol]

I couldn't find a way to linkify the logo so that it points to a specific URL. For example, this sample README.md contains a link:

# pkgdown <a href="https://pkgdown.r-lib.org/"><img src="man/figures/logo.png" align="right" /></a>

but the resulting html only contains:

<div class="page-header">
<img src="logo.png" class="logo" alt=""><h1 id="pkgdown-">pkgdown 
<a class="anchor" aria-label="anchor" href="#pkgdown-"></a>
</h1>
</div>

It's as if the link is stripped. It would be nice if it was respected or if there was some yaml setting to control the URL associated to the logo.

[hadley]

For various complicated reasons, the logo is stripped from the HTML and re-added by pkgdown to each page. Unfortunately I think it's going to be a lot of work to fix this, especially given that there's already a prominent link to the home page.