Add simple configuration file

This allows users to create simple configuration file
(`~/.config/bat/config` on Linux) that has the following format:

    ```bash
    --flag1
    --flag2
    --option1=value1
    # lines beginning with '#' are ignored
    --option2=value2
    # empty lines and trailing whitespace are also ignored

    --option3=value3
    ```

src/config.rs

@@ -0,0 +1,65 @@
+use std::ffi::OsString;
+use std::fs;
+
+use dirs::PROJECT_DIRS;
+
+pub fn get_args_from_config_file() -> Vec<OsString> {
+    let config_file = PROJECT_DIRS.config_dir().join("config");
+    fs::read_to_string(config_file)
+        .map(|content| get_args_from_str(&content))
+        .unwrap_or(vec![])
+}
+
+fn get_args_from_str<'a>(content: &'a str) -> Vec<OsString> {
+    content
+        .split('\n')
+        .map(|line| line.trim())
+        .filter(|line| !line.is_empty())
+        .filter(|line| !line.starts_with("#"))
+        .map(|line| line.into())
+        .collect()
+}
+
+#[test]
+fn empty() {
+    let args = get_args_from_str("");
+    println!("{:?}", args);
+    assert!(args.is_empty());
+}
+
+#[test]
+fn single() {
+    assert_eq!(vec!["--plain"], get_args_from_str("--plain"));
+}
+
+#[test]
+fn multiple() {
+    let config = "
+    -p
+    --style numbers,changes
+
+    --color=always
+    ";
+    assert_eq!(
+        vec!["-p", "--style numbers,changes", "--color=always"],
+        get_args_from_str(config)
+    );
+}
+
+#[test]
+fn comments() {
+    let config = "
+    # plain style
+    -p
+
+    # show line numbers and Git modifications
+    --style numbers,changes
+
+    # Always show ANSI colors
+    --color=always
+    ";
+    assert_eq!(
+        vec!["-p", "--style numbers,changes", "--color=always"],
+        get_args_from_str(config)
+    );
+}