Option for strand-aware expand

[emdann]

Following discussion with @ivirshup in #144, this is a proposed implementation for a strand-aware expand operation, adding the option direction_col.

[nvictus]

I migrated the strand-awareness logic to a new function bioframe.shift, basically following the proposal here.

Unlike expand, the semantics of shift always treats negative values as upstream and positive as downstream for both bounds of the ranges. expand itself could be deprecated.