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.

1 files changed, 89 insertions, 0 deletions

diff --git a/doc/aerc-templates.7.scd b/doc/aerc-templates.7.scd
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+aerc-templates(7)
+
+# NAME
+
+aerc-templates - template file specification for *aerc*(1)
+
+# SYNOPSIS
+
+aerc uses the go "text/template" package for the template parsing
+which supports basic go lang operations.
+
+# MESSAGE DATA
+
+The following data can be used in templates. Though they are not all
+available always.
+
+*Addresses*
+	An array of mail.Address. That can be used to add sender or recipient
+	names to the template.
+
+	- From: List of senders.
+	- To: List of To recipients. Not always Available.
+	- Cc: List of Cc recipients. Not always Available.
+	- Bcc: List of Cc recipients. Not always Available.
+	- OriginalFrom: List of senders of the original message.
+	  Available for quoted reply and forward.
+
+	Example:
+
+	Get the name of the first sender.
+	```
+	{{(index .From 0).Name}}
+	```
+
+	Get the email address of the first sender
+	```
+	{{(index .From 0).Address}}
+	```
+
+*Date and Time*
+	The date and time information is always available and can be easily
+	formated.
+
+	- Date: Date and Time information when the compose window is opened.
+	- OriginalDate: Date and Time when the original message of received.
+	  Available for quoted reply and forward.
+
+	The _dateFormat_ function can be used to format the date and time.
+
+	Example:
+
+	Format the date to go's time package format options.
+	```
+	{{dateFormat .Date "Mon Jan 2 15:04:05 -0700 MST 2006"}}
+	```
+
+*Subject*
+	The subject of the email is available for quoted reply and forward.
+
+	Example:
+	{{.Subject}}
+
+*Original Message*
+	When using quoted reply or forward, the original message is available.
+	It can be used using two functions that are available to templates.
+
+	Example:
+
+	_wrapText_ function can be used to wrap the original text to a number
+	of characters per line.
+	```
+	{{wrapText .OriginalText 72}}
+	```
+
+	_quote_ function prepends each line with "> " and wraps the text to
+	72 characters pre line.
+	```
+	{{quote .OriginalText}}
+	```
+
+# SEE ALSO
+
+*aerc*(1) *aerc-config*(5)
+
+# AUTHORS
+
+Maintained by Drew DeVault <sir@cmpwn.com>, who is assisted by other open
+source contributors. For more information about aerc development, see
+https://git.sr.ht/~sircmpwn/aerc.