Added CLI interface document.

cli/interface.txt

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+* Selection
+
+select (current)
+    artist      <id | name> 
+    album       <id | name>
+    playlist    <id | name>
+    file        <id | name>
+    tag(s)      <tag-names>
+
+* List
+
+list (all)
+    artists     <id | name>
+    albums      <id | name>
+    playlists   <id | name>
+    bookmarks   <id | name>
+    files       <id | name>
+    tag(s)      <id | name>
+
+* Playlist Addition
+
+add selection to playlist <id | name>
+add album <id | name> to playlist <id | name>
+add artist <id | name> to playlist <id | name>
+add file <id | name> to playlist <id | name>
+
+* Tagging
+
+tag selection <tags>
+tag album <id | name> <tags>
+tag artist <id | name> <tags>
+tag file <id | name> <tags>
+tag playlist <id | name> <tags>
+
+tag current
+    album <tags>
+    artist <tags>
+    file <tags>
+    playlist <tags>
+
+* Play Queue Modification
+
+enque selection
+enque album     <id | name>
+enque artist    <id | name>
+enque file      <id | name>
+enque playlist  <id | name>
+
+play selection
+play album      <id | name>
+play artist     <id | name>
+play file       <id | name>
+play playlist   <id | name>
+
+clear
+    queue
+    playlist
+    selection
+
+* Transport operations
+
+play
+pause
+stop
+rwd | rewind        [<amount>]
+ff | fastforward    [<amount>]
+prev    [<count>]
+next    [<count>]
+
+
+----
+
+top-level commands:
+    select
+    list
+    add
+    enque
+    tag
+    play
+    pause
+    stop
+    next
+    prev
+    ff
+    rwd
+    clear