Add feature to descend into subdir of parent dir

Instead of doing `cd ../../../parent/subdir` or `cd $(bd parent)/subdir`
you can do `bd parent/subdir` to get the same result.
The argument provided is split into a search string and a descend path.
The search string is used instead of `$2` (or `$1`) in the main code, so
`-s` and `-si` work the same way as before,
and the descend path is appended to `$NEWPWD` if match is found.

Note: the argument is split at the first `/` only so its possible
to do something like `bd parent/subdir/subdir2/subdir3`

Example:

You are in dir `/home/user/project/src/org/main/site/utils/file/reader/whatever` , and lets say `src` has another directory `com`.
you can easily go to `com` using `bd src/com`. No need to use `bd`'s output to run another `cd`. I mean that's the whole purpose of `bd` right?

You can also do `bd sr/com` or `bd SRc/com` if your alias uses `-s` or `-si` respectively.