implemented libofx_proc_string

implemented previously absent libofx_proc_string.  Refactored existing
file loading code to read the file into a string and then call
libofx_proc_string on it.

This allows clients to be able to parse ofx files without having to
write them out to disc.

The trick to getting OpenSP to interpret the arguments as  literal
strings instead of file names is to prefix the string data with
“<LITERAL>”.
don't display invalid line number info

Don’t display line/column information if we don’t have a line number
get account name information

parse the ofx ACCTINFO node and its DESC attribute in order to get the
bank’s name for the account.

When the ACCTINFO node is closed grab the last account in the
account_tree and call its add_accountinfo.  (Not sure if it’s possible
to have an ACCTINFO without an account added to account_tree.)

It’s not especially pretty but it works.