v0.2.7 release summary

[tlambert03]

v0.2.7 is a minor feature & bugfix release including:

• parameter value persistence: use @magicgui(persist=True) to store the values in the GUI to disk when they are changed, and restore them when the GUI is recreated in a future session (Persist parameter values across sessions #160).
• a preliminary Image widget magicgui.widgets.Image. Requires pip install magicgui[image] to work (Image widget #140)
• adds a widget_init parameter to magic_factory... useful for connecting events and such after a factory creates a new widget instance (Add widget_init parameter to magic_factory #159).
• fixes a bug when a parameter-less function is used with call_button=True (fix function with no params and callbutton #149)
• fixes FileEdit used with directory mode
• fixes a bug in Range/SliceEdits