Add custom printing formatting

[quasilyte]

Define -format argument that accepts a string that describes how match results should be printed. If it's an empty string, the entire match is printed as is (current behavior).
For non-empty patterns every match is formatted accordingly.

Example:

phpgrep -format '$s' srcdir 'die("s:str")'

pattern: 'die("s:str")'
format: '$s'

input: die("hello")
output without format: die("hello")
output with format: "hello"

[quasilyte]

Should think beforehand how it interacts with #25.

[JTBrinkmann]

Would be great if we could use named matchers in the output (e.g. phpgrep -format '{{.x}}' test.php '$foo = ${x}')

[quasilyte]

@JTBrinkmann it's doable.
I'll figure something out in the near time.

[quasilyte]

Let's open a separate issue for this and close this one. I forgot to close this issue when -format was implemented.

[quasilyte]

Feel free to test it. :)
I added a few examples to the README.
The syntax is exactly identical to what you proposed.