Highlighting tab errors

Added option `hltaberrors` which helps to spot sloppy whitespace errors
with tabs used instead of spaces or vice versa.

It uses the value of `tabstospaces` option as a criterion whether a
tab or space character is an error or not.
If `tabstospaces` is on, we probably expect that the file should contain
no tab characters, so any tab character is highlighted as an error.
If `tabstospaces` is off, we probably expect that the file uses
indentation with tabs, so space characters in the initial indent part
of lines are highlighted as errors.

internal/config/settings.go

@@ -289,6 +289,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"fileformat":      defaultFileFormat(),
 	"filetype":        "unknown",
 	"hlsearch":        false,
+	"hltaberrors":     false,
 	"incsearch":       true,
 	"ignorecase":      true,
 	"indentchar":      " ",