Provide persistent config for xtask

[IsaacWoods]

Provide a config file to set things like the default architecture for the xtask

Things to configure:

• Architecture you're developing for
• Profile to compile in (i.e. default vs release) (maybe per component so you can have e.g. a fast seed but fast turnaround on kernel)
• Userspace tasks to include in the default config (at some point we need to link this all up inside seed so we can change this properly dynamically)
• List of crates for things like clean targets, Gource, documentation, etc. Thinking you add an entry for each crate with options for whether to document it etc.