Bugfixes (#13)

* several fixes:

- fix #9 + #10: switched to koanf module and dropped support for HCL
- fix #11: disabling colors on windows
- fix #12: fixed race condition in go routine call inside for loop,
  images had been downloaded multiple times
- remove hcl support and use toml format (same thing, better parser)
- update documentation and example config on TOML format of config file
- use Config as arg instead of singular args
- use x/errgroup instead of sync.Waitgroup inside image download loop

---------

Co-authored-by: Thomas von Dein <tom@vondein.org>

example.conf

@@ -0,0 +1,30 @@
+#
+# kleingebaeck sample configuration file.
+# put this to ~/.kleingebaeck.
+#
+# Comments start with the '#' character.
+
+# kleinanzeigen.de user-id. must be an unquoted number
+user = 00000000
+
+# enable verbose output (same as -v), may be true or false.
+# other values: notice or debug
+loglevel = "verbose"
+
+# directory where  to store downloaded  ads. kleingebaeck will  try to
+# create it. must be a quoted string.
+outdir = "test"
+
+# template for  stored adlistings.  To enable  it, remove  the comment
+# chars up until the last #"""
+#template="""
+#Title: {{.Title}}
+#Price: {{.Price}}
+#Id: {{.Id}}
+#Category: {{.Category}}
+#Condition: {{.Condition}}
+#Created: {{.Created}}
+
+#{{.Text}}
+# """
+