Implement syntax mapping

This adds a `-m`/`--map-syntax` option that allows users to (re)map
certain file extensions or file names to an existing syntax.

For example:
```
bat --map-syntax .config:json
```

The option can be use multiple times. Note that you can easily make
these mappings permanent by using `bat`s new configuration file.

closes #169

src/syntax_mapping.rs

@@ -0,0 +1,35 @@
+use std::borrow::Cow;
+use std::collections::HashMap;
+
+#[derive(Debug, Clone)]
+pub struct SyntaxMapping(HashMap<String, String>);
+
+impl SyntaxMapping {
+    pub fn new() -> SyntaxMapping {
+        SyntaxMapping(HashMap::new())
+    }
+
+    pub fn insert(&mut self, from: String, to: String) -> Option<String> {
+        self.0.insert(from, to)
+    }
+
+    pub fn replace<'a>(&self, input: &'a str) -> Cow<'a, str> {
+        let mut out = Cow::from(input);
+        if let Some(value) = self.0.get(input) {
+            out = Cow::from(value.clone())
+        }
+        out
+    }
+}
+
+#[test]
+fn basic() {
+    let mut map = SyntaxMapping::new();
+    map.insert("Cargo.lock".into(), "toml".into());
+    map.insert(".ignore".into(), ".gitignore".into());
+
+    assert_eq!("toml", map.replace("Cargo.lock"));
+    assert_eq!("other.lock", map.replace("other.lock"));
+
+    assert_eq!(".gitignore", map.replace(".ignore"));
+}