Http-prompt

- New feature: Shell substitution (54, 61, 74)
- New feature: Piping to shell (61, 74)
- New feature: Be able to delete all parameters of a given type (79)

- New feature: env command (70, 73)
- New feature: output redirection (70, 73)
- New feature: source and exec commands (70, 73)
- Remove grouped-by-hostname persistent contexts (70, 73)
- Change default URL to http://localhost:8000 (78)
- Change persistent context format from JSON to "http-prompt script" (78)

- Fix bug: Syntax error on option names with dashes (69)
- Allow to set cookies automatically (46)
- Add contributing guide (62)