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/main.rs

@@ -23,9 +23,11 @@ extern crate wild;
 mod app;
 mod assets;
 mod clap_app;
+mod config;
 mod controller;
 mod decorations;
 mod diff;
+mod dirs;
 mod inputfile;
 mod line_range;
 mod output;