Cleaner errors, implement output package.

[wjdp]

• Calls to package specific checkErr methods replaced with single CheckErrorPanic() method.
• Addition of CheckErrorGeneric(), same as CheckErrorPanic but without the panic call, prints error and calls os.Exit(1).
• Addition of AbortWith method, handles exiting nicely -- print red and exit with os.Exit(1). CheckErrorGeneric calls this.
• Replacement of common errors (missing CLI args, missing config file) calling panic -> CheckErrorGeneric or AbortWith.
• Will close CLI: Replace panic calls with more user friendly messages #17.

Example when calling program without path or config file.

~ ❯❯❯ htmltest
No path provided & .htmltest.yml does not exist.
See htmltest -h for usage.

[codecov-io]

Current coverage is 95.03% (diff: 75.00%)

Merging #29 into master will increase coverage by 0.55%

@@             master        #29   diff @@
==========================================
  Files            16         16          
  Lines           833        826     -7   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits            787        785     -2   
+ Misses           33         30     -3   
+ Partials         13         11     -2   

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