Add config file with support for custom commands and git repos, fix #1

src/config.rs

@@ -0,0 +1,31 @@
+use directories;
+use failure;
+use std::collections::BTreeMap;
+use std::fs;
+use toml;
+
+#[derive(Deserialize, Default)]
+pub struct Config {
+    commands: Option<BTreeMap<String, String>>,
+    git_repos: Option<Vec<String>>,
+}
+
+impl Config {
+    pub fn read() -> Result<Config, failure::Error> {
+        let base_dirs = directories::BaseDirs::new();
+        let config_path = base_dirs.config_dir().join("topgrade.toml");
+        if !config_path.exists() {
+            return Ok(Default::default());
+        }
+
+        Ok(toml::from_str(&fs::read_to_string(config_path)?)?)
+    }
+
+    pub fn commands(&self) -> &Option<BTreeMap<String, String>> {
+        &self.commands
+    }
+
+    pub fn git_repos(&self) -> &Option<Vec<String>> {
+        &self.git_repos
+    }
+}