Line numbers for debug mode

[gdevenyi]

Possible implementation here:
https://coderwall.com/p/wil8da/debug-bash-scripts-with-line-numbers-more-info

[virgilwashere]

Expanding upon #103, I've finished the code portion of adding colour and line numbers to debug mode.

SC2235 is a legit suggestion/critique

Incidentally, this PR passes shellcheck linting on main.sh so that should help close off #107. Will add another PR for updating shellcheck installation in .travis.yml

All current tests are passing on 👷 TravisCI.

PR coming next, and then I plan to add ✅ tests to cover the expected output.

Virgil

[virgilwashere]

@kvz any idea what went wrong with the 🍏osx tests here: https://travis-ci.com/virgilwashere/bash3boilerplate/builds/117169882 ??

🐧linux passed!

This should be the extent of the changes...

dist: xenial
addons:
 homebrew:
    update: true
    packages:
      - coreutils
      - gnu-sed

and the homebrew is just moving to addons: from before_install:

[kvz]

Left a comment in that PR 👌