Convert spaces<->tabs ignores "tab stops"

[giseburt]

The convert spaces to tabs and tabs to spaces commands ignore the tab stop positions that Atom itself honors.

For example, with tabs set to 4-spaces wide, if the line is (with spaces represented as . and tabs as -):

A-  gap-here

Should translate to:

A...gap.here

But instead tabs are blindly replaced with a set number of spaces, making the translation (wrongly):

A....gap....here

Which then changes where the characters line up in Atom, when it shouldn't.

Going from spaces to tabs is similarly wrong, where it will only replace runs of spaces that are a full tab width (4, in this example), regardless of where they are. They should also honor the tab stop positions.

I created pull request #67 with the necessary code fixes and spec updates.