Cache management

[seabbs]

At the moment the cache is automatically deployed to the directory in which function are run. This is not good practice as the user should have a say in any files being saved to their computer. It does offer some nice benefits in that data is stored between R sessions (which is good when doing a lot of things in bash).

An alternative might be to default to set the cache as a temporary folder but to provide an option to set the cache globally. This could either be done using a variable for each function (which would get tedious to set each time) or using a function that sets the cache and then saves its location to a global variable. A Sys.getenv call could then be the default for the cache argument (similar to how slackr works) in each function falling back to a tmp cache if not found.

What do people think?