.add_argument() should also add kwargs to callsites

[ajberchek]

.add_argument() should also have the ability to add kwargs to callsites to perform codemods where a callsite's kwarg differs from the function definition's default for that arg.

The doc for this specific modifier has been updated to remove the distinction between positional and keyword args since both types can be added to the function definition and the callsites in this PR.

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[amyreese]

I'm not certain what this is getting us. If you're adding a keyword argument to the definition, there's no need to add it to the callers (in the current implementation) because the value will be the same. If the goal is to be able to use the is_call() and is_def() filters to make this actually useful (one pass to add argument to def, second pass to add argument to callers), then we should also improve this modifier to look for cases where these arguments already exist.

[ajberchek]

It is the latter case.

The specific way I used this during my codemods was to add a value, different than the function's default, to a callsite and avoid needing to provide the positional args in between.

Good point about looking for arguments that already exist; I will update this PR to account for that.

[ajberchek]

Functionality has been added to avoid adding an argument if it is already present in function definitions, and callsites for both positional and kwarg.

[thatch]

I squashed the history a little, this should be in a state to review now @jreese

[thatch]

This is on top or PR 102, I believe.