v0.2.7

- Added support for optional placeholders.
  ```bash
  jf "{str_or_bool: %?(str)q %?(bool)s, optional: %?(optional)q}" str=true
  # {"str_or_bool":"true","optional":null}
  ```

- Added support for placeholders with variable length values.
  ```bash
  jf [%*s] 1 2 3
  # [1,2,3]

  jf {%**q} one 1 two 2 three 3
  # {"one":"1","two":"2","three":"3"}
  ```

- Generate manpage.