Pass an optional buffer to `yt-setup-buffer'
Allow strings to be passed to yt-buffer-setup

Insert result if action is callable, else insert it as it is.
Separate buffer to show issue info along with list
Show issue detail in its own buffer with RET key
Make issue db optional to status buffer
Revert "Allow strings to be passed to yt-buffer-setup"

This reverts commit c25bc9298397b9812bbdbcdf90edbd8e48c9739b.
Show individual commit in a dedicated buffer
Switch to new buffer *AFTER* computing contents

Often the context where a method is called is important. For example the
line in which <return> is pressed in a issue listing is required to know
the line number and then the corresponding issue.

Switching to the new buffer before the contents are computed kills the
context, and is not cool. Compute the required string in the
context *BEFORE* the switch and then just dump the string as it is.

This also simplifies the defun `setup-buffer' which had to anticipate
arguments of type string, as well as functions.

Credits to @aaditya