Faster elfeed-search-update

[shitikanth]

Save the value of the corresponding elfeed-entry in text-property and only call elfeed-search-print-entry-function when the entry has changed or we have encountered a new entry.

On my system, with ~7000 elfeed-search-entries this reduces the time of elfeed-search-update with a small number of changes from ~0.70s to ~0.03s.

[shitikanth]

In current state, if window-width or any parameter for the elfeed-search-print-entry-function has changed in the meantime, the un-updated rows and updated rows will look inconsistent. For such a situation, I have added an option for a 'reset argument which clears the buffers before proceeding.