feat: added get tasks command

Added a get tasks command, which lists all the project's tasks by
module.

docs/reference/commands.md

@@ -336,6 +336,21 @@ Outputs the status of your environment.
 
     garden get status 
 
+### garden get tasks
+
+Lists the tasks defined in your project's modules.
+
+
+##### Usage
+
+    garden get tasks [tasks] 
+
+##### Arguments
+
+| Argument | Required | Description |
+| -------- | -------- | ----------- |
+  | `tasks` | No | Specify task(s) to list. Use comma as a separator to specify multiple tasks.
+
 ### garden init
 
 Initialize system, environment or other runtime components.

docs/reference/config.md

@@ -692,6 +692,11 @@ module:
       # Required.
       name:
 
+      # A description of the task.
+      #
+      # Optional.
+      description:
+
       # The names of any tasks that must be executed, and the names of any services that must be
       # running, before this task is executed.
       #

examples/tasks/hello/garden.yml

@@ -19,5 +19,8 @@ module:
   tasks:
     - name: node-migration
       command: [knex, migrate:latest]
+      description: Creates the users table.
       dependencies:
         - postgres
+    - name: foo-task
+      command: [echo hello]

examples/tasks/user/garden.yml

@@ -13,6 +13,7 @@ module:
   tasks:
     - name: ruby-migration
       command: [rake, db:migrate]
+      description: Populates the users table with a few records.
       dependencies:
         # node-migration creates the users table, which has to exist before we use
         # ruby-migration to insert records into it.