add key selector argument to replace

[mattlqx]

this adds a -s/--key-selector argument to the replace command. in non-regexp mode, it will specify a complete key to match against. in regexp mode, if the key selector yields any matches it will be considered be successful. in this case, replacements will be evaluated against that key-value pair. the key selector match will also be highlighted in the display.

this pull also fixes two issues:

• improper evaluation of the add/remove markup on a line diff during replace
• indexes being generated for regexps (index+length) did not match the indexes being generated for substrings (start&end) resulting in a panic for matches longer than half the string

after confirmation:

[fishi0x01]

Fixes #72

As usual, this looks amazing @mattlqx. Thank you :bow:

One little remark: Codeclimate is complaining about formatting. Can you run gofmt -w cli/search.go to format the code?