Support line continuations in strings

[chatura-atapattu]

Strings used in attributes for build rules that contain commands may use line continuations to format the command for readability. Currently, these multiline commands are unwrapped into one line with poor spacing.

Example:

genrule(
    name = "foo",
    srcs = [],
    outs = ["foo.h"],
    cmd = "./$(location create_foo.pl) \
                  > \"$@\"",
    tools = ["create_foo.pl"],
)

becomes

./$(location create_foo.pl) 	> "$@"

genrule(
    name = "foo",
    srcs = [],
    outs = ["foo.h"],
    cmd = "./$(location create_foo.pl)                   > \"$@\"",
    tools = ["create_foo.pl"],
)

We should maintain these line continuations such that if a backslash is followed by a newline, this is not stripped as it currently is.

[vladmos]

Could you please also add a test for the formatter? build/testdata/060.in would be a good place (in the "correct escape sequence" block). Thanks