Manage 'type' option

[Enchufa2]

Enabling bspm should set

options(pkgType="both")

Then we should honor the type option:

• At the very least, bspm should always fallback to install.packages() if type="source". This would help with eddelbuettel/r-ci/issues/10.
• When #41 is implemented, then this could behave as on Windows, i.e., in interactive sessions, whenever there's a source version that is newer than the binary, the user is asked whether to install this one. Not sure what happens in non-interactive sessions though.