Handling default/none values in theme config

[zwpaper]

After #452 merged, lsd now supports yaml-based theme configuration.

but a completed configuration file is needed, if any items are missed, the whole theme would fall back to the default one.

we should update the theme struct to Option and allow some items missed.

we should discuss that the missed fields should indicate which one color:

1. the default foreground color, like no color
2. the default theme color, falls back the missed fields to the default item.
3. the default error color, could be used by invalid items

[meain]

I have not been able to take a look at the PR, but being able to skip some would be necessary for it to be useful IMO. My idea is that users should be able to ignore some options and that would use the default color, but at the same time they should be able to specify something like None if they wanna keep it as no color(default terminal foreground).

[zwpaper]

Yes, it is necessary! but the theme feature could work functionally without it, so that people could use the feature much earlier if we merge #452 first.

because

1. we can always add the ability to skip without breaking any currently implemented functions.
2. supporting one of the options would be an easy job, but it is necessary to support both.
3. I don't know when did I have time to implement the ability to skip 😂

[meain]

Makes sense ;). Let me go through #452 over the weekend.