Fix single-line characterwise put.

Apparently forgot to implement single-line characterwise puts. Added some
implementation. Also changed the registers schema (again). Previously
each register object contained a list of objects which had properties
"index" and "content" to represnt the original index of the yanked
characters/line and the content itself, respectively. Now each register just
contains an array of lines instead, since there doesn't seem to be a use
case for storing the original line index.

OLD
---
Vim.get('registers')['"'];
=> {
    type : 'linewise',
    text : [
        { content : 'hi', index : 1 },
        { content : 'by', index : 2 }
    ]
}

NEW
---
Vim.get('registers')['"'];
=> {
    type : 'linewise',
    text : [ 'hi', 'by' ]
}