Add description of awk command

Added a description of how to use awk for basic column extraction from a file.
Fix typo in last example for awk

The last example in the awk description tried to print the 4th column with '-' as the delimiter. This should have been the 3rd column
Add self to contributions.rst
Add more examples and clean code-blocks in awk

Added a couple of more advanced examples in the awk section. Also replaced code blocks with .. code-block:: console
Merge pull request #131 from jivid/awk-decription

Add description of basic awk usage in shell_tools