Add Templates with Parsing

+ Changes NewComposer to return error.
+ Add lib to handle templates using "text/template".
+ Add -T option to following commands
    - compose.
    - reply
    - forward
+ Quoted replies using templates.
+ Forwards as body using templates
+ Default templates are installed similar to filters.
+ Templates Config in aerc.conf.
    - Required templates are parsed while loading config.
+ Add aerc-templates.7 manual for using template data.

templates/forward_as_body

@@ -0,0 +1,2 @@
+Forwarded message from {{(index .OriginalFrom 0).Name}} on {{dateFormat .OriginalDate "Mon Jan 2, 2006 at 3:04 PM"}}:
+{{wrapText .OriginalText 72}}

templates/quoted_reply

@@ -0,0 +1,2 @@
+on {{dateFormat .OriginalDate "Mon Jan 2, 2006 at 3:04 PM"}}, {{(index .OriginalFrom 0).Name}} wrote:
+{{quote .OriginalText}}