Add "trim path" option which can be used to relocate sources.

When pprof is asked to show annotated source for a profile collected on
another machine for a Go program, the profile contains absolute paths
which may not exist on the local machine. In that case pprof currently
fails to locate the source with no option to help it. The new option
adds a way to specify one or several source path prefixes that should be
trimmed from the source paths in the profile before applying the search
using search path option.

For example, taking the example from the issue where the source file
path in the profile is
/home/teamcitycpp/agent09/work/56cbaf9067/_gopath/src/badoo/lakafka/main.go
and the local path is /home/marko/lakafka/main.go. The user may
specify
`-trim-path=/home/teamcitycpp/agent09/work/56cbaf9067/_gopath/src/badoo`
to make pprof find the source. The source path doesn't need to be
specified if the current working dir is anything at or under
`/home/marko/`.

When the trim path is not specified, it is guessed heuristically based
on the basename of configured searched paths. In the example above,
setting `-search-path=/home/marko/lakafka` would be sufficient to
activate the heuristic successfully. Or having the current directory as
`/home/marko/lakafka` since the search path is by default set to the
current working directory. Note that the heuristic currently
does not attempt to walk the configured search paths up like the search
does. This is to keep it simple, use `-trim-path` explicitly in more
complicated cases.

Fixes #262.