fix condition for the rest of input

the input must rest the length of a string to be peeked at least.
fixes parse error at embedded documents by string eval, and `-e`
command line options.
fix embedded documents

tabs are allowed after `=begin` and `=end`.
raise `SyntaxError` if no =end is found.
fix char after `=end`

skips() advance after the string skipped.
skip to the next line and reset column after `=end`
count skipped line numbers
Merge pull request #2161 from nobu/embedded_document

Embedded document