feat: add CLI_ARGS placeholder in list output

[notnmeyer]

in the --list output when a task uses CLI_ARGS but args are not provided insert a placeholder.

for example,

➜ tsk --list -f examples/tasks.toml -F template
template:
  commands:
    echo {{.CLI_ARGS}}

but if args are provided, use them,

➜ tsk run -- --list -f examples/tasks.toml -F template -- BLARG
template:
  commands:
    echo BLARG

closes #85