Check for _mini

In the editor.Editor.set_tab_usage definition, the code would check to see if self._mini evaluates to True.  This was a problem if self._mini had not been created yet, such as when opening a new file.  As I'm working on a Go project right now and don't feel like switching editors for it, I had to fix that in order to be able to open files in that project - because Go's style guide recommends tabs (and, in fact, gofmt converts spaces to tabs).
Slight change from how I fixed it; put the tab initialisation after the minimap initialisation
Merge pull request #1272 from nelsam/tab-support

Check for _mini
Closes issue #1270
Merge pull request #1294 from diegosarmentero/master

 Closes issue #1270
Closes #1240
Merge pull request #1295 from diegosarmentero/master

 Closes #1240
Fixed Issue #1234
Merge pull request #1296 from diegosarmentero/master

 Fixed Issue #1234
Fixed Issue #1235
Merge pull request #1297 from diegosarmentero/master

 Fixed Issue #1235
Fixed Issue #1233
Merge pull request #1298 from diegosarmentero/master

Fixed Issue #1233
Fixed Issue #1210
Merge pull request #1299 from diegosarmentero/master

 Fixed Issue #1210