Code blocks: support specifying a starting line number

[damithc]

Suggestion: for code blocks, support specifying a starting line number.
Reason:

• When there are multiple code blocks in a near vicinity, this feature will allow the a code block line numbers to continue from the previous code block. e.g., first code block has lines 1-5, the second one can be set to start from 6.
• Also, it is useful when quoting a subset of lines from a longer code block that is already in the page (the sub set of lines can be set to follow the same line numbers they had in the longer code block.

[damithc]

IIRC, I think this was originally suggested by @alyip98 (?) somewhere in an earlier thread.

[acjh]

Yes, in #991 (comment).