Improve indentation for multi-line expressions

When someone used a multiline string:

    cmd: "..." +
         "...",

bpfmt used to print this out as:

    cmd: "..." +
    "...",

This change doesn't do the quote alignment like I see in some of our
user-created instances of this, but it does indent a single level:

    cmd: "..." +
        "...",

Test: unit tests
Test: Compared bpfmt results before and after across AOSP
Change-Id: I61bf790be9d08a187857b2725facf71e8b38e372