Allow to pass indent_length to json_pp

[bbkr]

Default 3 spaces indentation is pretty unusual.
Commonly used indentation in modern editors is 4 spaces, and they become confused when opening file formatted by json_pp.

Please allow to modify this value in --json_opt, for example:

cat foo.json | json_pp --json_opt utf8,indent_length=4

Thanks!

[charsbar]

Thanks for the request. Implemented.

[bbkr]

Awesome, thanks!