Espeak supports Speech Synthesis Markup Language (SSML), and you can just put those tags in the middle of a plain text file (i.e. you do not need to convert the whole thing to proper SSML). The tag you are looking for is probably
<emphasis level="...">. Other possible tags are listed here: http://espeak.sourceforge.net/ssml.html
Espeak will not use them by default, you will need to pass the
-mflag. For example, you could use
espeak -m -f your_file.txt
The documentation is rather terse, so you might have to dive into source code in order to find things like possible values. (I would write better documentation, but there are nine year old unmerged patches and the source repository is unavailable, so it seems the project is completely dead.) The possible values for the emphasis tag (and some other tags) can be found here on Github. They are:
Did you mean the <emphasis level="strong">green</emphasis> beans? Note that it will not work without quotes (whereas
<emphasis level=strong> would work in HTML, it does not work without quotes in SSML/espeak).
Alternatively, espeak can read words in full caps differently by using the
-k flag: if you use the command
espeak -k20 -f your_file.txt, you'll notice that it changes the pitch for words in "FULL CAPS". The two options do not mix, so you cannot have emphasis and a pitch change by using both options.
In SSML, however, you can mix them:
Did you mean the <emphasis level="strong"><prosody pitch="75">green</prosody></emphasis> beans? will be read with both emphasis and a higher pitch.