Allow for setting (but not unsetting!) variables

As described in #31. This commit is feature incomplete.

src/config_file.rs

@@ -49,6 +49,7 @@ pub(crate) enum EnvVariableOption {
 pub(crate) enum EnvVariableValue {
     String(String),
     Array(Vec<String>),
+    Set(bool),
 }
 
 #[cfg(test)]

src/main.rs

@@ -233,6 +233,8 @@ fn to_shell_source(vars: &EnvironmentVariables, shell: &Shell) -> String {
         // Any arrays are treated as a path.
         let (value, is_path) = match raw_value.clone() {
             EnvVariableValue::String(string) => (expand_value(&string), false),
+            EnvVariableValue::Set(true) => ("1".to_string(), false),
+            EnvVariableValue::Set(false) => continue, // todo!
             EnvVariableValue::Array(array) => {
                 let v_expanded: Vec<String> =
                     array.iter().map(|value| expand_value(value)).collect();