mounts: Add name mapping config option

[bep]

I'm doing some foundational improvements to Hugo, and I stumbled over some limitations to single file mounts (as in: Not a directory). These mounts were intended to be used for directories, that the file mounts worked was a little accidental. One use case would be the below:

[module]
  [[module.mounts]]
    source = "README.md"
    target = "content/_index.md"

There are some limitations to the above, and it also makes the mount code unnecessarily complex. So, with the work I'm doing right now I'm rewriting the above to be a directory mount with a source filter (README.md) and a name map (README.md<=> _index.md).

I have not exposed this feature in the config yet, but I suspect it would be really useful, as described in #4691

I'm not sure how it would look, maybe:

[module]
  [[module.mounts]]
    source = "." # Project root
    target = "content"
    includeFiles = "**/*.md"
     [[module.mounts.renames]]
     "README.md".to "_index.md"

[markphilpot]

I don't know if this is the right place to show support for this, but I have an additional use case that I think this would solve and right now is a blocker for me to switch to Hugo.

I use Ulysses external folder support for my writing, and they use textbundle format for their "posts" -- http://textbundle.org

The spec uses text.md as the markdown container within the textbundle and assets/* for relative images.

Since the only way hugo resolves relative images is through a leaf bundle (which currently requires index.md or _index.md) this means that images are broken in those posts when rendered in hugo.

My blog has a mix of index.md leaf bundles and Ulysses textbundles, so it would ultimately need to support both (not just a simple config option telling hugo which file to look for in the leaf bundle)