Introduce `cabal install`

[nattybear]

Some beginners may not know how to use cabal install.

Add some explanation about cabal install at 5.4. Fancy options parsing before running command hs-blog-gen --help.

[soupi]

Thanks for the report. This issue makes sense to me, however I would prefer not to cover cabal install in the book, and instead describe how to find the executable so it can be run directly, and also remind that one can use stack/cabal commands we learned previously to run the program.