Add 'erc-version' and use it to display ERC version consistently

Add 'erc-version' and use it to display ERC version consistently

* lisp/erc/erc.el (erc-version): New constant holding the current ERC
version, now used in the function with the same name to produce a
version string for use across ERC for consistency.  Also, add another
optional argument, 'bold-erc', which when non-nil, marks the "ERC"
portion of the string with the control character for bold display.
(erc-quit/part-reason-default): Use the 'erc-version' function for a
consistent version string.
(erc-cmd-SV): Mention the ERC version number from the 'erc-version'
constant.
(erc-ctcp-query-VERSION): Use the 'erc-version' function for a
consistent version string.