tools/pipbot: Refactor show() error path

Issue #204.
tools/pipbot: Print usage if 'show' arg is unkown

Issue #204.
tools/pipbot: Refactor for get_configure_args()

Issue #204.
tools/pipbot: Add the ability to show configure args

If pipbot is invoked with 'show <target> <build>' arguments, then the
output is the list of configure arguments for that configuration. Eg:

$ ./pipbot show local debug
./configure \
        --disable-minify-css \
        --disable-minify-html \
        --disable-minify-js \
        --enable-content-hashing \
        --enable-debug \
        --enable-documentation \
        --enable-git-hooks \
	--enable-tools \
	--enable-install \
	--disable-authentication \
	DB_HOST='localhost' \
	DB_NAME='pipdb' \
	DB_PASSWORD='mypassword' \
	DB_USERNAME='username' \
	WWW_ROOT='/var/www'

Issue #204.
Merge branch 'feature/204'