Add .cabal/bin to user PATH

Installing cabal-install for the first time should add .cabal/bin to the user PATH (.bash_profile) or at least prompt the user and ask if it should do so.

My reasoning: some cabal packages (haskell-src-exts) depend on installed haskell programs (happy) and will not find those it they are not in the PATH.

Start by tracing the first-time cabal-install setup and how user configuration such as .bash_profile is handled. Done means .cabal/bin is added to the user PATH or the installer clearly prompts whether to add it, while preserving the reported ability to find installed Haskell programs.

Written by the indexing model from the issue text.