2014/5/11: new read system spec.
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Reading a journal from some data source conceptually consists of:

1. Parse the data records into fields providing some or all of the standard
   journal transaction fields - at least date, description and amount.

2. Expand (if needed) these partial journal transactions into
   complete ones.

In practical terms, it happens in one of these ways:

1. the data source is a file or stdin

2. 
   If FILE.rules (or other file specified with --rules-file)
   exists, it can define rules which help with parsing. Eg the skip,
   fields, and date-format rules.

2. Expansion: partial transactions are fleshed out into complete ones.
   Eg partial transactions from CSV records need to have an account and
   a balancing posting added. Expansion is done in several ways:

   1a. Rules: if FILE.rules (or other file specified with --rules-file)
       exists, it can define rules which help with expansion. Eg field
       assignments and conditional blocks.

       Pro: easy, somewhat backward compatible, built in, cross platform.
       Con: limited flexibility.

   1b. Filter: or, if FILE-read (or other file specified with --read-filter)
       exists, it is used as a filter to translate FILE into (partial) journal
       format, which is then parsed with the (partial) journal reader.
       Pro: powerful, flexible.
       Con: requires programming & tools, data is parsed twice.

   2a. History: if a transaction is still not complete, the best recent match
       for it among existing transactions is used as a template to fill out
       missing fields/postings (as with hledger add or ledger-autosync).
       Pro: no rules or programming required, learns from past manual corrections.
       Con: less precise, more likely to require manual correction, requires existing data.

   2b. Guess: or, if there is no existing data or no acceptable match (or
       history matching has been disabled with --no-history-match), we guess
       default values for the missing fields.