Develop mechanism for user's to define overridden functions

[gothub]

Currently recordr defines overridden functions in maskedFunctions.R (maybe need name change?). Develop a way for users to have another way to override functions that they wish to record provenance for, maybe base R functions recordr doesn't override by default, or functions from other libraries (i.e. ggsave() from ggplot2). One approach is to have a 'library' of overriding functions under the ~/.recordr directory, that are loaded when startRecord() is called.