Fix #287

[fox0430]

#287

[jrfondren]

Crash remains:

1. start moe without a file argument
2. i
3. 1
4. <RET>
5. 2
6. <ESC>
7. v
8. k
9. d

This leads (by step 6) to a buffer consisting of

1 1
2 2 <CURSOR>

And crashes with:

moe.nim(51)              moe
moe.nim(40)              main
normalmode.nim(538)      normalMode
editorstatus.nim(261)    resize
editorstatus.nim(211)    writeStatusBar
macros.nim(469)          writeStatusBarNormalModeInfo
gapbuffer.nim(87)        []
system.nim(3879)         failedAssertImpl
system.nim(3872)         raiseAssert
system.nim(2918)         sysFatal
Error: unhandled exception: /Users/jfondren/nim/moe/src/moepkg/gapbuffer.nim(87, 11) `0 <= index and index < gapBuffer.size` Gapbuffer: Invalid index. [AssertionError]

Additionally, this PR prevents deletion of empty lines. In a buffer that looks like (with line numbers):

1
2
3
4 <CURSOR>
5
6

You can't enter vkd to delete lines 3 and 4 like you could in previous versions of moe.

[jrfondren]

Crash remains:

start moe without a file argument

1. o
2. 1
3. <ESC>
4. v
5. k
6. d

This leads (by step 3) to a buffer consisting of

1
2 <CURSOR>1

And crashes with:

moe.nim(51)              moe
moe.nim(40)              main
normalmode.nim(538)      normalMode
editorstatus.nim(261)    resize
editorstatus.nim(211)    writeStatusBar
macros.nim(469)          writeStatusBarNormalModeInfo
gapbuffer.nim(87)        []
system.nim(3879)         failedAssertImpl
system.nim(3872)         raiseAssert
system.nim(2918)         sysFatal
Error: unhandled exception: /Users/jfondren/nim/moe/src/moepkg/gapbuffer.nim(87, 11) `0 <= index and index < gapBuffer.size` Gapbuffer: Invalid index. [AssertionError]