global variables in workflow.yaml

ran across a situation in one of the examples when it would have been convenient to have some global variables available in the `command` jinja templates. Perhaps we could accept two types of YAML formats, one that just lists tasks, and another that is an object with key:values that are global variables to use throughout the workflow, but with a special key called `tasks` (or something) that would be the list of tasks. Something like this:

```yaml
---
x_col: 1
y_col: 2
tasks:
  # process the results
  -
  creates: data/x_y.dat
  depends: data/results.tsv
  command: cut -f{{x_col}},{{y_col}} {{depends}} > {{creates}}

  # create a figure for the correlation between x and y
  -
  creates: data/x_y_correlation.png
  depends: 
    - src/scatterplot.py
    - data/x_y.dat
  command: python {{depends|join(' ')}} {{creates}}

  # create a figure to display the distribution of x
  -
  creates: data/x_pdf.png
  depends: 
    - src/pdf.py
    - data/x_y.dat
  command: python {{depends|join(' ')}} {{x_col}} {{creates}}

  # create a figure to display the distribution of y
  -
  creates: data/y_pdf.png
  depends: 
    - src/pdf.py
    - data/x_y.dat
  command: python {{depends|join(' ')}} {{y_col}} {{creates}}
```