Implement parselet capture variable $ as a dict

[phorward]

Regarding #96, the following options would be nice as well:

• The capture variable $ becomes a dict, where elements with the same key are chained . Such behavior is implemented in phorward/plist, for example.
• The capture variable $ may also be used alone or with item get/set, likewise $["x"] and $[1], additionally to $x, $1.

[phorward]

#114 and commit 440c648 makes the $0 variable not part of the stack anymore. This might be the initial start to resolve this issue.

As this issue eliminates the current capture concept, we should think about these bullet points as well:

• Maybe the $0 variable can be held with every Frame, which fixes the collision problem
• This issue and a comment posted in Establishing generic parselets and functions #105 (Think about a replacement of Capture by RefValue with a new value type Capture, so that Repeat returns a capture range and not a list of values) would make Captures easier to maintain, as Ranges are abstracted by RefValues. This is only an option to think about later.