Add a new function in order to be able to set the number of spaces for indentation

[jdespatis]

This new function doesn't change current behavior of pretty-data,
but let user change this setting with the following code:

var pretty = require('pretty-data').pd;
pretty.indent(4);

[jdespatis]

@vkiryukhin If you feel this PR is not complete, do not hesitate to tell me, so that I can add those things to make the merge possible

The aim of this PR is to let current behavior unchanged, and add the possibility for the developer to tune the number of spaces for indentation

Do not hesitate to give your feedback
Thanks for your lib

[jdespatis]

@vkiryukhin any feedback for this merge ?

[bjrmatos]

this is cool! @vkiryukhin can you merge it?