Added TASKFILE_DIR template variable

[Shivakishore14]

Added TASKFILE_DIR template variable to access the directory where the task file is located.

example:

version: '2'

tasks:
    build: 
        cmds:
            - echo '{{TASKFILE_DIR}}'

output

$ task build
echo '/Users/coda/go/src/github.com/go-task/task/cmd/task'
/Users/coda/go/src/github.com/go-task/task/cmd/task

closes #215

[andreynering]

Hi @Shivakishore14, and thanks for opening this PR.

Unfortunately, there are use cases which won't properly work with this patch.

1. If someone use --dir to run a Taskfile in another directory, this will return the wrong directory
2. This will also return the wrong directory for included Taskfiles
3. Probably other similar use cases

I think this will be more easily done correctly if added as a variable instead of a function.

So, since I realized this isn't as simple, I just removed the good first issue from #215 and will close this for now.

Feel free to ping me if you have any questions or comments. 🙂