Allow to use other glyphs for trees

[memeplex]

Given that you intentionally are using just a handful of glyphs (as discussed in #262) could you:

1. Just call propertize by hand instead of using the all-the-icons wrapper. This is a simple change, the wrapper is handy but it's not a big deal anyway.

2. Allow the user to provide a string/list/alist with the few glyphs you have to set.

I'm requesting this because many users probably have Nerd fonts or similar installed in their system and the dependency on all-the-icons is a source of potential conflicts, I guess. And even if it isn't it would be cleaner to provide 5 or 6 fonts without the need to install 6 font families first (btw, the all-the-icons installer is broken).

I don't mean to remove all-the-icons support altogether, of course, it's a good fallback. Just to give a hook to users (probably the ones on arch linux) already using glyphs packages of their liking.

[memeplex]

Just in case the above is not clear, the intention is to resolve the glyph in the list provided by the user and, when unable to do so, to fetch the corresponding all-the-icons glyph as a fallback. Then apply propertize on the resolved glyph. So the all-the-icons machinery will be fetching fonts by name without applying further properties.