message: Fix build by changing Option field
  Introduced by commit an Option modifier on Context field in the
  Shell struct.
diff: @@ -99,7 +99,7 @@ pub struct Shell<'a> {
  /// Contains a list of built-in commands that were created when the program started.
  pub builtins: &'a FnvHashMap<&'static str, Builtin>,
  /// Contains the history, completions, and manages writes to the history file.
  - pub context: Option<Context>,
  + pub context: Context,
  /// Contains the aliases, strings, and array variable maps.
  pub variables: Variables,
  /// Contains the current state of flow control parameters.
  