Add single_indent option

[belka-ew]

Since the DStyle proposal to indent continuation lines by two tabs instead of
one was rejected (see dlang/dlang.org#2766), and it is
the only behaviour dfmt supports, I suggest to add an option to indent
everything with a single tab. So the option changes the indentation inside the
parens.

Double indentation is useful to visually emphasize a code block after an
argument list or if-, while-conditions and so on. For example

if (true
  && true)
  doSomething();

vs

if (true
        & true)
    doSomething();

The indentation in the second case makes it clear where the condition ends.

Some style guides enforce:

• curly braces even in single line if-statements
• curly braces on the new line

In the case of a such style guide, the problem disappears because there is
always a new line between the condition and the code block:

if (true
    && true)
{
    doSomething();
}

It also eliminates the need to think about aligning the code with mixed parens,
brackets and braces (e.g. ([])), just indent everything with one tab.

[WebFreak001]

requesting additional review from Hackerpilot because the indentation engine is black magic to me

[belka-ew]

Can we get this futher? Last week I've tested this option on a bunch of Funkwerk projects, hundreds of thousands of lines of code, and it works as expected with a few exceptions caused by existing indentation bugs.