cmd/godoc: show examples in text mode

cmd/godoc: show examples in text mode

Added the command line flag -ex to godoc to print examples in
text output.

Samples from the generated output:

$ godoc -ex strings Index
...
func Index(s, sep string) int
    Index returns the index of the first instance of sep in s, or -1 if sep
    is not present in s.

    Example:
        fmt.Println(strings.Index("chicken", "ken"))
        fmt.Println(strings.Index("chicken", "dmr"))
        // Output:
        // 4
        // -1
...

$ godoc -ex container/heap
...
package heap
    import "container/heap"

    Package heap provides heap operations for any type that implements
    heap.Interface. A heap is a tree with the property that each node is the
    minimum-valued node in its subtree.

    Example:
        // This example demonstrates an integer heap built using the heap interface.
        package heap_test

        import (
            "container/heap"
            "fmt"
        ...

    Example:
        // This example demonstrates a priority queue built using the heap interface.
        package heap_test

        import (
            "container/heap"
            "fmt"
        )
...

Fixes #3587.

R=golang-dev, minux.ma, adg, rsc, gri
CC=golang-dev
https://golang.org/cl/7356043