Add option to set an absolute directory path

[leonvogt]

This PR will introduce the option to set an absolute directory path.

Before - For example, a Redis accessory directory like:

• data:/data would create the directory $PWD/my-appname-redis/data
• /var/lib/redis:/data would create the directory $PWD/my-appname-redis//var/lib/redis

After - With this change, a Redis accessory directory like:

• data:/data would create the directory $PWD/my-appname-redis/data
• /var/lib/redis:/data would create the directory /var/lib/redis

The decision on whether Kamal will create an absolute or relative directory is based on the presence of a leading / in the path.

Please note that this change may introduce a breaking change for some users...

resolves #453

[bendangelo]

Can this get reviewed again? I would really like this change.