fix: allow for zero backups and zero daysToKeep

[lamweili]

Fixes #59

Public APIs:

• RollingFileStream
backups excludes the hot file (original behavior retained)

  backups=0	backups=1 (default)	backups=2	backups=3
  filename.log filename.log.1	filename.log filename.log.1	filename.log filename.log.1 filename.log.2	filename.log filename.log.1 filename.log.2 filename.log.3

• DateRollingFileStream
daysToKeep excludes the hot file (original behavior retained)

  daysToKeep=0 (default)	daysToKeep=1 (default)	daysToKeep=2	daysToKeep=3
  filename.log filename.log.2022-01-11 filename.log.2022-01-10 filename.log.2022-01-09	filename.log filename.log.2022-01-11	filename.log filename.log.2022-01-11 filename.log.2022-01-10	filename.log filename.log.2022-01-11 filename.log.2022-01-10 filename.log.2022-01-09

Private APIs (only if used directly and not via public APIs):

• RollingFileWriteStream
numToKeep excludes includes the hot file (new behavior)

  numToKeep=0	numToKeep=1	numToKeep=2	numToKeep=3
  throws Error()	filename.log filename.log.1	filename.log filename.log.1 filename.log.2	filename.log filename.log.1 filename.log.2 filename.log.3

Important
It is also to note the file does not roll within itself (truncate its older entry for newer entry).
It truncates all and appends only the new entry.

var rollers = require('streamroller');
var stream = new rollers.RollingFileStream('myfile', 6, 0);
stream.write("abc"); // add as first row
stream.write("def"); // add as second row
stream.write("ghi"); // truncate all and add as first row
stream.end();

Output:

myfile   - ghi