Modify .gitignore.

The new approach is to whitelist directories and
files we want in the repository. This way 'git status'
will not flood output with untracked files.

.gitignore

@@ -1,35 +1,42 @@
-*~
-.DS_Store
-*.orig
-deb/.vagrant
-dist/*
-MANUAL.*
+*
+!.circleci/*
+!.editorconfig
+!.gitattributes
+!.github/*
+!.gitignore
+!.hlint.yaml
+!.mailmap
+!.stylish-haskell.yaml
+!AUTHORS.md
+!BUGS
+!CONTRIBUTING.md
+!COPYING.md
+!COPYRIGHT
+!INSTALL.md
 !MANUAL.txt
-.configure-stamp
-.cabal-sandbox
-cabal.sandbox.config
-pandoc.cabal.orig
-man/man?/*.1
-man/man?/*.5
-man/man?/*.html
-*.diff
-*.o
-*.hi
-*.pyc
-.hie/
-.policeman-evidence/
-.projectile/
-.vscode/
-/COPYING.rtf
-/COPYRIGHT.txt
-/cabal-dev/
-/windows/*.msi
-/windows/*.wixpdb
-windows/*.wixobj
-data/reference.docx
-data/reference.odt
-.stack-work
-/stack.yaml.lock
-cabal.project.local
-/dist-newstyle/
-pandoc-2*/
+!Makefile
+!README.md
+!README.template
+!RELEASE-CHECKLIST
+!SECURITY.md
+!Setup.hs
+!cabal.project
+!changelog.md
+!default.nix
+!pandoc.cabal
+!release.nix
+!shell.nix
+!stack.yaml
+!app/*
+!benchmark/*
+!citeproc/*
+!data/*
+!doc/*
+!linux/*
+!macos/*
+!man/*
+!src/*
+!test/*
+!tools/*
+!trypandoc/*
+!windows/*