Add indentation to json dump method

[sina-rostami]

The problem is specified in issue #747 .

This is a simple solution to improve the dump method without breaking any APIs and is fully backward-compatible.

[gittiver]

Have a look at my comment. As a general comment I tend to leave dump and dump internal as it is and put the indentation into a pretty_print function or something like this.

[sina-rostami]

Have a look at my comment. As a general comment I tend to leave dump and dump internal as it is and put the indentation into a pretty_print function or something like this.

Thank you for the review. Leaving those functions as they are, I should duplicate a bunch of code in pretty_print as they have much in common. Moreover, dump and dump_internal can still be used with and for the non-pretty style purpose without losing any performance in comparison to the current version. However, if my reasons didn't convince you, please let me know and I'll separate them.

[mrozigor]

I don't know if formatting output should be a part of the library. But that's up too you . Code looks ok I guess ;)