Allow for project argument in check and update commands

[ben-edna]

When having more projects, it is easier to allow for single project actions:

For instance if you have a manifest with multiple projects, of which one is named cpputest:

dfetch check cpputest
dfetch update cpputest

A similar mechanism was already implemented in #78, but this uses a named argument, _ think a positional argument is the most natural in this case. But also allow for multiple projects:

dfetch check myproject myotherproject yet-another-project