Add option to do custom code formatting

[jakemac53]

No description provided.

[jakemac53]

Updating this to instead take String formatCode(String generatedCode) argument so you can do whatever you want, including just returning the original string.

[natebosch]

Who is asking for this? Is the motivation a performance improvement?

[jakemac53]

The motivation is performance primarily yes. The angular compiler creates some code which dartfmt takes a long time to figure out (hundreds of milliseconds on some real examples).

In that case the solution was to increase the line length, but we opted for a more general solution which allows any custom formatting of code.