Custom formatter

[Odonno]

First of, thank you for this library. It really works like a charm.

I used this library to retrieve and store data for a .Xrc file. Kind of like you may have seen a .npmrc with npm. My concern is that the output formatting I expect is a little different than the current output from this lib.

I would like to go from this:

[section]
key=value

To this:

[section]
    key = "value"

Notice the subtle differences:

• The value has double-quotes around it
• Each key-value pair inside a section is indented
• The spaces between "=" and key/value

[zonyitoo]

The space around = is already added by https://github.com/zonyitoo/rust-ini/pull/93 . So we may need an indentation configuration in WriteOption.

I am busy these days, feel free to open a PR if you will. :)