Fix clippy reported problems of single_char_pattern lint

This change fixes occurrences flagged by the clippy lint
`single_char_pattern`. E.g.,

> warning: single-character string constant used as pattern
>  --> src/print.rs:6:29
>   |
> 6 |         if self.0.ends_with("\n") {
>   |                             ^^^^ help: try using a char instead: `'\n'`
>   |
>   = help: for further information visit
>     https://rust-lang-nursery.github.io/rust-clippy/master/index.html#single_char_pattern

Author: d-e-s-o

src/parser.rs

@@ -192,7 +192,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 loop {
                     match self.iter.peek() {
                         None => { break; }
-                        Some(arg) if arg.starts_with("-") => {
+                        Some(arg) if arg.starts_with('-') => {
                             break;
                         }
                         Some(value) => {

src/print.rs

@@ -3,7 +3,7 @@ use action::{IFlagAction, ParseResult};
 
 impl IFlagAction for Print {
     fn parse_flag(&self) -> ParseResult {
-        if self.0.ends_with("\n") {
+        if self.0.ends_with('\n') {
             print!("{}", self.0);
         } else {
             println!("{}", self.0);