Add custom templates

[Hugovdberg]

This is a first try at creating custom templates, as proposed in #204 and, originally, in #36. It seems like a very large change, but in fact the changes can be grouped in a few categories:

• create.template.R was added as a lightweight wrapper around create.project with some checking for a sensible location
• A global option ProjectTemplate.templatedir was introduced to allow the user to specify a directory which contains her or his templates.
• migrate.template was added as a wrapper around migrate.project to migrate the configuration of the template.
• create.project was changed to take an argument template instead of minimal, which defaults to full to preserve the original behavior.
• Most tests had to be updated for the new signature of create.project. I removed all references to minimal = TRUE|FALSE because just a few tests actually depended on the template used in the test.
• The inst/default/full directory was moved to inst/default/templates/full, and a new minimal template was created at inst/default/templates/minimal to cope with the new template system.

Some stuff has yet to be added, such as documentation for the functions and for the website, but I wanted to show this already to get some feedback before doing so.

[KentonWhite]

@Hugovdberg There is a conflict in the inst/defaults/full/config/global.dcf. Can you resolve the conflict please?

[Hugovdberg]

I was looking into fixing the problem, but I see that the inst/defaults/config/default.dcf still has the old version number, and some other default settings than inst/defaults/templates/full/config/global.dcf. Shouldn't those two be the same? And if they should be the same, than how do you feel about removing the global.dcf from the base templates provided, and add some logic to create.project to copy the default.dcf into the project if it's missing in the template? That way we don't have to manage the default settings in both the full and minimal template.

[KentonWhite]

Good point about not having to update the global.dcf file in multiple places. I'm not sure how much effort it would be. Lets make resolve the conflicts on this PR and then open a new PR to copy to use a single global.dcf file.

[Hugovdberg]

Apparently default.dcf contains some other defaults than global.dcf, which the tests do rely on. I reverted those changes, though I'm not exactly sure why those settings were hardcoded in the tests and why the tests assume different settings than the normal defaults.

[Hugovdberg]

@KentonWhite @connectedblue did you get a chance to review this already? Is there anything holding us back from implementing this feature?

[KentonWhite]

Sorry thought I had merged this. Thanks for the reminder.