Add markdown and man page docs generation methods

This adds two new methods to the `App` struct:

- `ToMarkdown`: creates a markdown documentation string
- `ToMan`: creates a man page string

Signed-off-by: Sascha Grunert <mail@saschagrunert.de>

testdata/expected-doc-full.man

@@ -0,0 +1,80 @@
+.nh
+.TH greet(8) 
+
+.SH Harrison
+
+.SH NAME
+.PP
+greet \- Some app
+
+
+.SH SYNOPSIS
+.PP
+greet
+
+.PP
+.RS
+
+.nf
+[\-\-another\-flag|\-b]
+[\-\-flag|\-\-fl|\-f]=[value]
+[\-\-socket|\-s]=[value]
+
+.fi
+.RE
+
+
+.SH DESCRIPTION
+.PP
+app [first\_arg] [second\_arg]
+
+.PP
+\fBUsage\fP:
+
+.PP
+.RS
+
+.nf
+greet [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]
+
+.fi
+.RE
+
+
+.SH GLOBAL OPTIONS
+.PP
+\fB\-\-another\-flag, \-b\fP: another usage text
+
+.PP
+\fB\-\-flag, \-\-fl, \-f\fP="":
+
+.PP
+\fB\-\-socket, \-s\fP="": some usage text (default: value)
+
+
+.SH COMMANDS
+.SH config, c
+.PP
+another usage test
+
+.PP
+\fB\-\-another\-flag, \-b\fP: another usage text
+
+.PP
+\fB\-\-flag, \-\-fl, \-f\fP="":
+
+.SS sub\-config, s, ss
+.PP
+another usage test
+
+.PP
+\fB\-\-sub\-command\-flag, \-s\fP: some usage text
+
+.PP
+\fB\-\-sub\-flag, \-\-sub\-fl, \-s\fP="":
+
+.SH info, i, in
+.PP
+retrieve generic information
+
+.SH some\-command

testdata/expected-doc-full.md

@@ -0,0 +1,62 @@
+% greet(8) 
+
+% Harrison
+
+# NAME
+
+greet - Some app
+
+# SYNOPSIS
+
+greet
+
+```
+[--another-flag|-b]
+[--flag|--fl|-f]=[value]
+[--socket|-s]=[value]
+```
+
+# DESCRIPTION
+
+app [first_arg] [second_arg]
+
+**Usage**:
+
+```
+greet [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]
+```
+
+# GLOBAL OPTIONS
+
+**--another-flag, -b**: another usage text
+
+**--flag, --fl, -f**="": 
+
+**--socket, -s**="": some usage text (default: value)
+
+
+# COMMANDS
+
+## config, c
+
+another usage test
+
+**--another-flag, -b**: another usage text
+
+**--flag, --fl, -f**="": 
+
+### sub-config, s, ss
+
+another usage test
+
+**--sub-command-flag, -s**: some usage text
+
+**--sub-flag, --sub-fl, -s**="": 
+
+## info, i, in
+
+retrieve generic information
+
+## some-command
+
+

testdata/expected-doc-no-commands.md

@@ -0,0 +1,36 @@
+% greet(8) 
+
+% Harrison
+
+# NAME
+
+greet - Some app
+
+# SYNOPSIS
+
+greet
+
+```
+[--another-flag|-b]
+[--flag|--fl|-f]=[value]
+[--socket|-s]=[value]
+```
+
+# DESCRIPTION
+
+app [first_arg] [second_arg]
+
+**Usage**:
+
+```
+greet [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]
+```
+
+# GLOBAL OPTIONS
+
+**--another-flag, -b**: another usage text
+
+**--flag, --fl, -f**="": 
+
+**--socket, -s**="": some usage text (default: value)
+

testdata/expected-doc-no-flags.md

@@ -0,0 +1,47 @@
+% greet(8) 
+
+% Harrison
+
+# NAME
+
+greet - Some app
+
+# SYNOPSIS
+
+greet
+
+# DESCRIPTION
+
+app [first_arg] [second_arg]
+
+**Usage**:
+
+```
+greet [GLOBAL OPTIONS] command [COMMAND OPTIONS] [ARGUMENTS...]
+```
+
+# COMMANDS
+
+## config, c
+
+another usage test
+
+**--another-flag, -b**: another usage text
+
+**--flag, --fl, -f**="": 
+
+### sub-config, s, ss
+
+another usage test
+
+**--sub-command-flag, -s**: some usage text
+
+**--sub-flag, --sub-fl, -s**="": 
+
+## info, i, in
+
+retrieve generic information
+
+## some-command
+
+