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
Add Hash#delete_if
simply use `isfinite`
Fix comment of exc_inspect (and adjust spaces)
Merge pull request #2181 from suzukaze/add-hash_delete_if

Add Hash#delete_if
Merge pull request #2182 from cremno/use-isfinite

simply use `isfinite`
Merge pull request #2183 from yui-knk/fix-error-comment

Fix comment of exc_inspect (and adjust spaces)
Fix typo.
Add Hash comments
Merge pull request #2184 from yui-knk/fix-typo

Fix typo.
Merge pull request #2185 from suzukaze/hash-comments

Add Hash comments
Merge pull request #2161 from nobu/embedded_document

Embedded document
Add Hash#flatten
Merge pull request #2186 from suzukaze/add-hash.flatten

Add Hash#flatten
Add document of header searcher of compiler.
Merge pull request #2187 from take-cheeze/header_searcher_doc

Add document of header searcher of compiler.