printer: support previews for long lines

This commit adds support for showing a preview of long lines. While the
default still remains as completely suppressing the entire line, this
new functionality will show the first N graphemes of a matching line,
including the number of matches that are suppressed.

This was unfortunately a fairly invasive change to the printer that
required a bit of refactoring. On the bright side, the single line
and multi-line coloring are now more unified than they were before.

Closes #1078

src/args.rs

@@ -746,6 +746,7 @@ impl ArgMatches {
             .per_match(self.is_present("vimgrep"))
             .replacement(self.replacement())
             .max_columns(self.max_columns()?)
+            .max_column_preview(self.max_column_preview())
             .max_matches(self.max_count()?)
             .column(self.column())
             .byte_offset(self.is_present("byte-offset"))
@@ -1142,6 +1143,12 @@ impl ArgMatches {
         Ok(self.usize_of_nonzero("max-columns")?.map(|n| n as u64))
     }
 
+    /// Returns true if and only if a preview should be shown for lines that
+    /// exceed the maximum column limit.
+    fn max_column_preview(&self) -> bool {
+        self.is_present("max-column-preview")
+    }
+
     /// The maximum number of matches permitted.
     fn max_count(&self) -> Result<Option<u64>> {
         Ok(self.usize_of("max-count")?.map(|n| n as u64))