Additional file browser keyboard actions

## Actions

### Existing Actions (kept as is):

`h` - open with default open setting
`j` - move selection down
`k` - move selection up
`l` - open with default open setting
`Space` - open with default open setting
`CR` - open with default open setting
`Esc` - focus vim view

### New Actions:

`o` - open with default open setting
`t` - open in new tab
`s` - open in vertical split
`i` - open in horizontal split
`/` - search - select next node containing string
`m` - display node manipulation menu - `Actions: (a)dd (m)ove (d)elete (c)opy`
`n` - search (forward) using last search string
`N` - search backward using last search string

## Modes

Search (`/`) and menu (`m`) are modal actions. Feedback is displayed via the status bar. `Esc` can be used to cancel either mode and return to "normal mode". In normal mode `Esc` reassumes its normal function: focus vim view.

### Search Mode

Basic line editing is supported with feedback displayed via the status bar. Type `/`, enter a search string, and then type `CR` to perform the search.

### Menu Mode

`Actions: (a)dd (m)ove (d)elete (c)opy`

#### (a)dd

A node can be added by typing `a` and then entering the name of the node to add. names ending in `/` will create a directory.

#### (m)ove

`m` can be used to move/rename the selected node. The user is prompted if this action would clobber an existing file.

#### (d)elete

`d` deletes the selected node. A confirmation prompt is displayed and the user must type `y` to actually delete the node, or `n` to cancel. Directories can only be deleted if empty.

#### (c)opy

`m` can be used to copy the selected node to the named location. The user is prompted if this action would clobber an existing file.