add example and default of unique / one-off runner

Author: anna-singleton

examples/runners.toml

@@ -12,3 +12,8 @@ quit_fast=false
 name="echo var showcase"
 cmd="echo {myvar}"
 quit_fast=false
+
+[[runners]]
+name="unique"
+cmd="{command}"
+quit_fast=false

src/defaults.toml

@@ -3,8 +3,12 @@ name="rust"
 cmd="cargo run"
 quit_fast=false
 
-
 [[runners]]
 name="list"
 cmd="ls"
 quit_fast=false
+
+[[runners]]
+name = "unique"
+cmd = "{command}"
+quit_fast = false