Reload config on file name changes

As EditorConfig is wholly filename pattern based and has path dependent configuration, it makes sense to reload the config automatically when the file name and / or path changes, instead of letting the user figure out they need to issue an `:EditorConfigReload` command. For added awesomeness, this makes the plugin smarter than most Vim filetype plugins, which take their cue from Vim’s `:h filetype.txt` and rarely (if at all) watch rename operations.

The PR also cleans up the autocommand group code by removing the redundant group name arguments (these are not needed inside an `augroup name ... augroup END` block – see `:h augroup`).