Start from a template when pinning without metadata

should be quite helpful when creating new packages
Haha... to be honest I pondered for a while on this one, I even did a:
```
% git grep -h '^license:' |sort | uniq -c | sort -nr
    264 license: "Apache-2.0"
    103 license: "ISC"
     58 license: "MIT"
     40 license: "BSD3"
     37 license: "LGPL-2.1+ with OCaml linking exception"
     28 license: "LGPL-2.1 with OCaml linking exception"
     24 license: "LGPL-3.0 with OCaml linking exception"
     19 license: "GPL-3 with OCaml linking exception"
     18 license: "GNU Lesser General Public License version 3"
     15 license: "GPL-3"
     14 license: "WTFPL"
     14 license: "GNU General Public License version 3"
     12 license: "BSD"
     10 license: "GPL"
      9 license: "LGPL-2.0 with OCaml linking exception"
      8 license: "GNU Library General Public License version 2.1"
      8 license: "BSD-3-Clause"
      7 license: "LGPL 2.1"
      7 license: "GPL 3"
      6 license: "LGPL"
      6 license: "GNU Lesser General Public License version 2.1"
      5 license: "LGPL v3"
      5 license: "BSD-2-Clause"
      4 license: "Zlib"
      4 license: "LGPL-2.0 &\n   LGPL-2.1 with OCaml linking exception &\n   ISC"
      4 license: "GPL-3+"
      4 license: "GPL-2"
      3 license: "LGPL-2 with OCaml linking exception"
      3 license: "ISC license"
      3 license: "GPL v2+"
      2 license: "LGPL-v3+ with OCaml linking exception"
      2 license: "LGPLv2+ with exceptions and GPLv2+"
      2 license: "LGPLv2"
      2 license: "LGPL+static"
      2 license: "LGPL+linking exception"
      2 license: "LGPL-2+ with OCaml linking exception"
      2 license: "GPL v3"
      2 license: "GNU Library General Public License version 2"
      2 license: "GNU General Public License version 2"
      2 license: "CeCILL-C"
      1 license: "QPL-1.0 and LGPL-2 with exceptions"
      1 license: "Public Domain"
      1 license: "public domain"
      1 license: "modified BSD"
      1 license: "MIT/X11"
      1 license:  "MIT"
      1 license: "LGPL with OCaml linking exception"
      1 license: "LGPL-v2 with OCaml linking exception"
      1 license: "LGPL v2.1, or MPL v1.1"
      1 license: "LGPL v2"
      1 license: "LGPL + static linking exception"
      1 license: ["LGPL 2 or later"]
      1 license: "GPL v2"
      1 license: "GPL-2.0 with OpenSSL exception"
      1 license: "GPL-2.0"
      1 license: "GPL 2.0"
      1 license: "GNU Library General Public License version 2 with special exception (cf LICENCE file)"
      1 license: "GNU Lesser General Public License version 2 or later"
      1 license: "CeCILL-B v1"
      1 license: "CeCILL"
      1 license: "BSD-like"
      1 license: "BSD-3-clause with OCaml linking exception"
      1 license: "BSD-3-Clause, or LGPLv2.1+"
      1 license: "BSD 3-Clause"
      1 license: "Apache"
      1 license: "AGPL"
```

I put `ISC` at some point, but changed back because I was experimenting with OPAM itself and the linking exn stuff is much more annoying to type ;)
No strpng opinion on the matter though (_and_ if I could avoid getting run over by a license troll, all the better !)