Fix alpha when set from a .json

[fc-anjos]

When trying to set a color scheme using pywal as a module, such as in this example, alpha was not being correctly exported. In .Xresources, for instance, URxvt*background would be set with [100]#rrggbb. This pull request fixes this behavior by:

Specifying a default value for the cases when alpha is not in data:
pywal/theme.py

if "alpha" not in data:
    data["alpha"] = "100"

and setting util.Color.alpha_num from data:
pywal/export.py

util.Color.alpha_num = all_colors["alpha"]

These changes can be appreciated in their context in the diff.

Open to implement further changes if I am missing something.
Thank you.

[fc-anjos]

Closing this pull request until I am able to fix broken functionality due to the changes.