User configuration file for non-formatting options

[kamalmarhubi]

This came up in #856, and is related to #837.

There are two types of options in rustfmt.toml right now: those that are related to the formatting style and those that are not.

There are currently three non-formatting options:

• write_mode
• verbose
• skip_children

These make sense as command line flags, and as individual user settings that govern invocation. They do not make sense at the project level.

I propose adding a per-user configuration file for these settings, and removing them from the project-level rustfmt.toml. Let's call this ~/.rustfmtconfig for now; the file name and location can be bikeshedded later.

[kamalmarhubi]

@nrc on #856:

I think that to start with the global/per-user config file should be separate from the per-project file, i.e., have completely separate options with different names and the file should have a different name. Then we don't need to worry about whether the per-user or per-project file takes precedence, etc. We can just use it for stuff like this.