Enable input from stdin, such as pipe output
Added option for database dump
Improve detection of stdin input and add example, add 'common' format constant, add available variables for filters to doc
Remove -s option, as it is not really needed
Update README
Merge branch 'master' of https://github.com/mcortizo/ngxtop into mcortizo-master

Allow ngxtop to parse logs directly from stdin. Pre-defined common log format for Apache access log.
Fix checking file exist even for 'stdin' log.
Merge branch 'master' of https://github.com/lebinh/ngxtop