Indentation updates

[d-torrance]

This is an improved version of #18, which was eventually reverted (#22). The proposal is to keep the same basic behavior of indentation in M2-mode, but with a few improvements:

• M2-electric-tab now checks to see if it was called from newline, and if it was, replicates the behavior of M2-newline-and-indent. This means that we can deprecate M2-newline-and-indent and just use newline, which is great because it has a lot more features. For example, users can now turn off automatic indentation after pressing Return by disabling electric-indent-mode.
• Previously, we cleaned up the whitespace while indenting in one case (when the current indentation was further than it should have been and we had to move it back), but now we clean it up in all cases. So for example, pressing Tab a bunch of times on a blank line won't result in alternating tab characters and spaces, but a bunch of tab characters possibly followed by spaces.
• M2-electric-tab is now documented!

[d-torrance]

I'm not quite happy with this yet -- using this-command to detect whether we just started a new line doesn't seem correct. Converting to a draft for now.

[d-torrance]

I'm happy with this now.

When the user specifically presses Tab (which is bound to indent-for-tab-command in M2-mode and org-cycle when looking at a Macaulay2 source block in Org Mode -- but more commands can be added by customizing M2-insert-tab-commands), we get the classic M2-electric-tab behavior of inserting spaces when on a blank line or to the right of some text.

But when M2-electric-tab is called from any other command (e.g., from newline, newline-and-indent, org-return, etc.), we indent based on the depth of the parentheses, matching the behavior of M2-newline-and-indent.