output: remove --line-number-width flag

This commit does what no software project has ever done before: we've
outright removed a flag with no possible way to recapture its
functionality.

This flag presents numerous problems in that it never really worked well
in the first place, and completely falls over when ripgrep uses the
--no-heading output format. Well meaning users want ripgrep to fix this
by getting into the alignment business by buffering all output, but that
is a line that I refuse to cross.

Fixes #795

src/args.rs

@@ -56,7 +56,6 @@ pub struct Args {
     invert_match: bool,
     line_number: bool,
     line_per_match: bool,
-    line_number_width: Option<usize>,
     max_columns: Option<usize>,
     max_count: Option<u64>,
     max_filesize: Option<u64>,
@@ -191,8 +190,7 @@ impl Args {
             .only_matching(self.only_matching)
             .path_separator(self.path_separator)
             .with_filename(self.with_filename)
-            .max_columns(self.max_columns)
-            .line_number_width(self.line_number_width);
+            .max_columns(self.max_columns);
         if let Some(ref rep) = self.replace {
             p = p.replace(rep.clone());
         }
@@ -401,7 +399,6 @@ impl<'a> ArgMatches<'a> {
             ignore_files: self.ignore_files(),
             invert_match: self.is_present("invert-match"),
             line_number: line_number,
-            line_number_width: try!(self.usize_of("line-number-width")),
             line_per_match: self.is_present("vimgrep"),
             max_columns: self.usize_of_nonzero("max-columns")?,
             max_count: self.usize_of("max-count")?.map(|n| n as u64),