feat: add navigation over Dockerfile-based projects

[ivanilves]

This PR adds navigation over Dockerfile-based projects. As of release 0.3.X and later we use custom function to filter relevant project paths from the all existing Git repository paths. We pass a custom function to directory.Collect() as a parameter

How to use?

Execute echo "mode: dockerfile" > .travelgrunt.yml in the root of a Dockerfile [mono]repo and then read this chapter.

The purpose of this PR is to demonstrate - how [easy] is to add a new working mode to travelgrunt - will it be our Dockerfile, Golang source files, Makefile, files with some custom content/comments or any other beasts you spot in typical wild [mono]repo, where directory tree depth and number of subfolders is way to big to just cd productivelly. #fatigue

We can also think about "true" monorepos with very complex structures and implement more YAML-driven configs like:

mode: advanced

rules:
  - path: terraform
    include:
      - terraform
      - terragrunt
  - path: code
    include:
      - golang
      - php
      - nodejs
   - path: build
     include: [Makefile]

Latter is for the future releases, of course, just thinking loudly here 😇

[codecov]

Codecov Report

Merging #34 (3fa97e2) into main (8f95996) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #34   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          200       204    +4     
=========================================
+ Hits           200       204    +4     

Impacted Files	Coverage Δ
pkg/config/config.go	100.00% <100.00%> (ø)
pkg/config/include/include.go	100.00% <100.00%> (ø)

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