allow add_ip short form, resolves #42

[Risto97]

This pull request implements the short signature for add_ip() function.

Now there are 3 versions:

1. Full form

add_ip(ip
    VENDOR vendor
    LIBRARY lib
    VERSION 0.0.1
)

This translates into vendor::lib::ip::0.0.1 target

In this version, it is possible to omit any of the VENDOR, LIBRARY, VERSION arguments.
A special case of full form is when all of them are omitted.

add_ip(ip)

This translates into ip target

2. Short form

In the short form the VLNV format can be passed to add_ip().
Like this:
add_ip(vendor::lib::ip::0.0.1)

In this form it is important that the full form is passed, as if one of the tokens are missing, how can we know what is VENDOR, LIBRARY, IP, VERSION

[Risto97]

resolve #42
close #42