Share directory is now configurable via an environment variable: better self-containedness

With this commit, the `TBL_SHARE` environment var can change the default "/usr/share" prefix to something else. This is especially useful when running torbrowser-launcher without installing it system-wide.

refs https://github.com/micahflee/torbrowser-launcher/issues/75
refs https://github.com/micahflee/torbrowser-launcher/issues/77