When the mouse is enabled, clicking on a header field switches focus to
that field (likewise for the terminal).

%F now shows the auth name or recepient name/address if the
message is from you.

Adds a message indicating the user's ability to :save or :pipe a message
with an unsupported mimetype and also adds a selector widget (similar to
the tutorial).

The selector widget was previously defined in the account wizard module,
so this commit breaks it out into its own module to allow for re-use.

Further, modify the BeginExLine() function to take an argument that
pre-populates the command line, allowing functions to initiate an ex
command without executing it.

Closes #95.

When viewing a message part with no available filter, clicking the UI
would cause a nil pointer dereference because the MouseEvent was passed
to the PartViewer's `term` field, which does not exist in the case of an
absent filter.

+ 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.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

The filter command shells out and returns almost immediately. Call
Wait() so the filter process gets reaped. Prior to this patch, aerc
creates a zombie process for every email that is viewed.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

A pager is spawned every time an email is viewed but not killed off when
quitting the msgviewer, thus leading to process leakage. This patch
fixes this by adding a Close method to the msgview widget, which is
called in the close command.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

This reverts commit 1339faf7881f33762c6e0a4915404e362fc51de1.