pkg: add support for [a-z.+-~] characters on package version number string. * pkg/private/get_description.m: currently pkg enforces a x.y.z (major, minor, and patch) numeric style for the string representing a package version. In addition, it will silently append a ".0" if the string is missing the patch version which causes weird errors for downstream packagers. However, there seems to be no reason in the code for this limitation which prevents 'x.y.z+' or 'x.y.z-rcN'. A real limitation we will have is that version string will be used in the package filepath. This change just expands the valid characters for the version string. It does not change the requirement when installing packages with the -forge option though (but Octave forge packages only have the x.y.z version string anyway).

To cite GNU Octave in publications use:

  John W. Eaton, David Bateman, Søren Hauberg, Rik Wehbring (2015).
  GNU Octave version 4.0.0 manual: a high-level interactive language for
  numerical computations.
  URL http://www.gnu.org/software/octave/doc/interpreter/

A BibTeX entry for LaTeX users is:

  @book{,
    author    = {John W. Eaton, David Bateman, S\oren Hauberg, and Rik Wehbring},
    title     = {{GNU Octave} version 4.0.0 manual: a high-level interactive language for numerical computations},
    year      = {2015},
    url       = {http://www.gnu.org/software/octave/doc/interpreter},
  }

We have invested a lot of time and effort in creating GNU Octave, please cite it
when using it.  See also `citation pkgname' for citing Octave packages.