Allow managing pkgset under local folder.

While replacing pakcageset name with "--local", the packageset
information will be managed under some/local/repo/.gvm_local folder, and
add the root path of repository as the first path in GOPATH and PATH.
This makes the repository more portable since we don't store all the
go-installed package files under $HOME/.gvm .

The repository root path for gvm is definded as $PWD while
"pkgset create --local" . When "pkgset list" under some configured local
repository, it will list the root path as the first packageset, and
marked with "L".
add -w option to grep
Merge pull request #20 from t-k/grep-with-w-option

add -w option to grep
Change response to already installed version.
Fix issue with running update source but then not properly finding version. () <= subshell fail
Merge pull request #18 from COLDTURNIP/master

Store packageset data under local path.
Provide support for vendored native code workflow.

This commit introduces generated overlay filesystem support for
${PATH}, ${LD_LIBRARY_PATH}, and ${PKG_CONFIG_PATH} as well as
includes a working example of how to take advantage of this support
when integrating with vendored code.