Support subtasks and checklists within tasks

[Valentin-v-Todorov]

Break down complex tasks into smaller subtasks.

• Add 'tix subtask add <parent_id> '
• Show progress bars for parent tasks
• Support nested subtasks (up to 2 levels)
• Add checklist format in task notes

[vaishnavxd]

I would like to work on this issue for Hacktoberfest @Valentin-v-Todorov

[Valentin-v-Todorov]

@vaishnavxd Thanks for collaborating !

[skalwaghe-56]

Can I work on this? As the contributor hasn't raised any PR yet? @Valentin-v-Todorov

[parrth20]

@Valentin-v-Todorov I would like to work on this issue for Hacktoberfest

[Valentin-v-Todorov]

@parrth20 Issue is already taken

[vaishnavxd]

@Valentin-v-Todorov I have opened a pull request please review it.

[vaishnavxd]

#90