Initial append of pcomn_cmdline library sources
Add pcomn::valtype_t<T> - metafunction to calculate "core" type stripped of references and cv-qualifiers
Allow to register CommandSuite commands in fully-declarative fashion

PCOMN_REG_SHCMD
Fix assertion_traits<basic_strslice<C> >
Add explicit conversion operator to std::string from pcomn::strslice