Change snippet indent to \t

Snippets indented with spaces always expand with the same tab length as the snippet itself. When using `\t` instead, atom will automatically replace `\t` with the preferred tab length specified in the user's configuration. It also actually replaces the tabs with spaces if soft tabs is enabled (it is enabled by default). A quick search confirms that most atom-language plugins do this.
This lets anyone use the snippets effectively no matter what their preferred indent style is.