Add ExtraCalldepth to Logger

[giftig]

ExtraCalldepth is added to the default calldepth of 2 to allow the
correct function name to be shown when exposing logger methods at
package level or wrapping them in utility functions

Add an example of how this works to examples/example.go
Update examples/example.go to include a new screenshot
Update README.md to include the new example.go

[giftig]

Hi @op I've added documentation per your request; I made an attempt at adding a test, but since the memory backend seems to ignore the calldepth, my approach didn't seem to achieve much, as Formatted(calldepth) is called in the tests.

This attempt, for example, doesn't achieve much as it passes for any value of ExtraCalldepth:

func TestExtraCalldepth(t *testing.T) {
    backend := InitForTesting(DEBUG)

    // Just output the shortfunc; that's all we care about in this test
    backendFormatter := NewBackendFormatter(
        backend,
        MustStringFormatter("%{shortfunc}"),
    )
    SetBackend(backendFormatter)

    log := MustGetLogger("test")
    wrappedError := log.Error
    log.ExtraCalldepth = 1
    wrappedError("")

    if "TestExtraCalldepth" != MemoryRecordN(backend, 0).Formatted(0) {
        t.Error("Unexpected shortfunc name")
    }
}

If you have any suggestions as to a better test strategy, let me know and I'll give it another whack; I'm inexperienced with testing in golang, I'm afraid.

[giftig]

I thought I'd note that this change was caused by me copying the updated contents of example.go directly into the code block; it looks like it was updated in an earlier commit but not amended in the code block in README.md

[giftig]

Looks like the example doesn't work in go v1... since this isn't a compulsory feature, though, I'd suggest support for >= 1.1 is better than nothing. The v1.1-only part could be split into a secondary example and excluded from the 1.0 build maybe?

[jbenet]

+1 to ExtraCalldepth

[op]

All code looks good! I will merge this, but will keep the ExtraCallDepth away from the normal example. This seems like an advanced feature that will confuse first time users. Thanks!