Add a ta config set subcommand

This uses `toml_edit` to edit the config file in-place.  For the moment,
it only supports top-level arguments, but can be extended to do other
things later.

cli/src/invocation/cmd/config.rs

@@ -0,0 +1,62 @@
+use crate::argparse::ConfigOperation;
+use crate::settings::Settings;
+use termcolor::{ColorSpec, WriteColor};
+
+pub(crate) fn execute<W: WriteColor>(
+    w: &mut W,
+    config_operation: ConfigOperation,
+    settings: &Settings,
+) -> anyhow::Result<()> {
+    match config_operation {
+        ConfigOperation::Set(key, value) => {
+            let filename = settings.set(&key, &value)?;
+            write!(w, "Set configuration value ")?;
+            w.set_color(ColorSpec::new().set_bold(true))?;
+            write!(w, "{}", &key)?;
+            w.set_color(ColorSpec::new().set_bold(false))?;
+            write!(w, " in ")?;
+            w.set_color(ColorSpec::new().set_bold(true))?;
+            writeln!(w, "{:?}.", filename)?;
+            w.set_color(ColorSpec::new().set_bold(false))?;
+        }
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::test::*;
+    use std::fs;
+    use tempfile::TempDir;
+
+    #[test]
+    fn test_config_set() {
+        let cfg_dir = TempDir::new().unwrap();
+        let cfg_file = cfg_dir.path().join("foo.toml");
+        fs::write(
+            cfg_file.clone(),
+            "# store data everywhere\ndata_dir = \"/nowhere\"\n",
+        )
+        .unwrap();
+
+        let settings = Settings::load_from_file(cfg_file.clone(), true).unwrap();
+
+        let mut w = test_writer();
+
+        execute(
+            &mut w,
+            ConfigOperation::Set("data_dir".to_owned(), "/somewhere".to_owned()),
+            &settings,
+        )
+        .unwrap();
+        assert!(w.into_string().starts_with("Set configuration value "));
+
+        let updated_toml = fs::read_to_string(cfg_file.clone()).unwrap();
+        dbg!(&updated_toml);
+        assert_eq!(
+            updated_toml,
+            "# store data everywhere\ndata_dir = \"/somewhere\"\n"
+        );
+    }
+}

cli/src/invocation/cmd/mod.rs

@@ -1,6 +1,7 @@
 //! Responsible for executing commands as parsed by [`crate::argparse`].
 
 pub(crate) mod add;
+pub(crate) mod config;
 pub(crate) mod gc;
 pub(crate) mod help;
 pub(crate) mod info;