Add ob repl -c/--config option

[TravisCardwell]

This option allows users to insert configuration into the .ghci file that is loaded by the REPL. It is up to the user to not insert commands that breaks things.

Example user .ghci file:

:set prompt      "\ESC[32;1mλ: \ESC[m"
:set prompt-cont "\ESC[32;1m : \ESC[m"
:set +t
:set +m
:set -XOverloadedStrings
:set -XScopedTypeVariables
:set -XTypeApplications

Example usage:

ob repl -c .ghci

I have:

• Based work on latest develop branch
• Followed the contribution guide
• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• Run the test suite: $(nix-build -A selftest --no-out-link) NOTE A number of tests fail in develop, before making changes. The ob repl tests do not, however, and these tests still pass with this commit.

  ob repl
    accepts stdin commands
    works with custom Prelude
  

• Updated the changelog
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[ali-abrar]

Thanks