Handle Multi-line selections for Bold and Italicize keyboard shortcuts

[CFiggers]

Currently, Ctrl+b and Ctrl+i simply surround the current selection with * and _, respectively. However, this does not work when multiple lines are selected. Instead, each line should independently have * or _ added to the beginning and end. E.g.:

// Before:

Multiple
Lines

// Current After Ctrl+B with both lines selected

*Multiple
Lines*

// Desired After Ctrl+B with both lines selected

*Multiple*
*Lines*