support customizing logo and favicon

[superyyrrzz]

#873
@chenkennt @vwxyzh @hellosnow @ansyral @qinezh

[hellosnow]

[roji]

This is great, but note that it still imposes class="svg" (I'm not sure what that means in your CSS and whether it's possible to put in a png etc.).

One idea, probably in addition to this PR, is to extract the logo into its own separate partial, allowing people to override it with whatever they want without having to duplicate the rest of the navbar.

[superyyrrzz]

@roji We use svg class to provide hover effect in docfx.css. Putting a png here can also work. BTW, it's nice to have a logo partial.

[ansyral]