Support escaped selectors and document it

[Chris00]

In view of this issue about selectors containing a colon, it would be nice to make sure that lambdasoup supports selectors with escape sequences and to document it (so it is clear to the users).

[aantron]

Lambda Soup now supports escape sequences in selector identifiers. In particular, see this new test for escaped colons:

lambdasoup/test/test.ml

Line 179 in b53b69f

test "#colon\\:" 1;

lambdasoup/test/pages/quoted.html

Line 17 in b53b69f

<p id="colon:"/>