feat: improve watch execution mode

The `watch` command now requires user action to move to the next
exercise.

BREAKING CHANGE: this changes the behavior of `watch`.

Cargo.toml

@@ -10,6 +10,7 @@ indicatif = "0.9.0"
 console = "0.6.2"
 notify = "4.0.0"
 toml = "0.4.10"
+regex = "1.1.6"
 serde = {version = "1.0.10", features = ["derive"]}
 
 [[bin]]
@@ -18,3 +19,4 @@ path = "src/main.rs"
 
 [dev-dependencies]
 assert_cmd = "0.11.0"
+glob = "0.3.0"