Release tarballs missing libshared

[vrusinov]

Steps to reproduce:

1. Download tarball from either taskwarrior.org (https://taskwarrior.org/download/tasksh-1.2.0.tar.gz) or from github releases tab.
2. tar xzf tasksh-1.2.0.tar.gz; cd tasksh-1.2.0; cmake .

Expected result:

cmake works

Actual result:

CMake Error at src/CMakeLists.txt:26 (add_library):
  Cannot find source file:

    libshared/src/Color.cpp

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx

It appears that libshared is not included in either download (likely because it's a submodule).