Instead of providing a set of macros that major modes use to define functions, we should provide a basic framework that major modes register with.

• Instead of one before-save function per mode, there should be a single function in langfmt, that calls the correct formatting function based on the active major mode
• Instead of one *-show-errors variable per major mode, there should be a single variable for langfmt
• Major modes should provide a default format-command, which the user should be able to customize (e.g. to use goimports instead of gofmt). Customization variables will live in per-major subgroups, and the subgroups will live in a langfmt group.