feature-request: run a command IF a condition passes

just like **[LINK](https://github.com/anishathalye/dotbot#format)** directive, there should one for running individual commands for SHELL directive.

Ex:
```yaml
- shell:
- command: apt update && apt upgrade -y
if: lsb_release -i | grep -io 'debian'
description: Update APT package repository

- command: dnf update -y
if: lsb_release -i | grep -io 'fedora'
description: Update DNF package repository
```

## why
- provides a better flow of running dotbot process
- can contain similar commands in a single shell directive.
- Ex: shell directive for updating repositories
- Ex: installing a package using package manager in one OS, downloading a script from another OS